Color wheels provide a way to describe the ordering of hue and, in some cases, to aid in understanding color mixing. The artist’s color wheel (Figures 1 and 2), epitomized by Itten [23], is used extremely widely in teaching. Its primary colors are red, yellow, and blue. This is the color wheel that students meet in primary school. In this wheel, the opposite of blue is orange. When students meet more advanced material in color theory, they find apparent contradictions. The printer’s color wheel has primaries cyan, magenta, and yellow, which the student might be taught to understand as a refinement of blue, red, and yellow. But curiously, for the students the color labeled “blue” in the printer’s color wheel is opposite to yellow, not to orange. In my own early introduction to color, I found the art books’ insistence that orange was the opposite of blue conflicted with my observation that, in many works of art and design, yellow appeared to be the more apposite opposite. Further confusion comes to the student when they meet the opponent-color theory, in which there are four principal colors, with blue opposite yellow (Figure 3); and Munsell’s color system in which there are five principal hues, with blue opposite yellow-red (Figure 4). The challenge for the educator is in explaining these differences.

These differences can be downplayed in educational material. For example, one text for art students states that “Color wheels must always have an even number of hues and that number must be divisible by three. Any other combination would not be a true and accurate color wheel” [5, p. 66, emphasis mine]. This is a simplification by the author for the benefit of the students, as the author is well aware of the NCS and Munsell color systems which have four and five principal colors, respectively, [5, p. 31] and which have a well-defined notation for describing colors around the hue wheel.

One of the reasons to question the received wisdom is that almost all art texts, inspired ultimately by Itten’s seminal work [23], use angles on the color wheel to determine “harmonious” color combinations (Fig. 2). If the color wheel is not immutable, as the different color wheels suggest, then these harmonies rest on insecure foundations.

This is by no means a new problem [4, Ch. 6B] [7]. Some of the differences between the different color wheels can be explained from the principles underlying their constructions and the uses for which they are designed. There is a difference in how you construct your color space depending on whether you are mixing colored lights, mixing colored pigments, or dealing with human visual perception [9]. For example, Itten’s artist’s color wheel is based on subtractive color mixing of pigments; opponent-color theory is based on visual perception; and Munsell aimed to bring clarity to color communication by establishing an orderly system for accurately identifying all colors. All the color spaces discussed in this paper are ways of specifying or mixing colors, so all can be considered as ways of dealing with pigment.

The contribution of this paper is to argue that our understanding of color wheels is mediated by the terms we use to describe colors, in particular, in the use of basic color terms [3]. This leads to some of the apparent differences between color wheels in two ways. First, generally across all color wheels, we use the same basic color name, such as blue, to represent subtly different colors in different wheels (Section 5), which confuses the student. Second, and specific to the artist’s color wheel (Fig. 1), while the artist’s (RYB) and printer’s (CMY) color wheels should both be identical, because they are both subtractive color mixing models, I argue that the differences between them are largely owing to the artist’s color wheel being actively driven by basic color terms in a way that puts it at odds with the optimal physical color mixing embodied in the printer’s color wheel (Section 6).

I first give a summary of the history of color spaces and color wheels (Section 2), then a history of color naming and an outline of Berlin and Kay’s theory of basic color terms (Section 3). I describe five of the most commonly used color wheels (Section 4). I demonstrate that imprecise use of color names explains a substantial amount of the apparent inconsistencies between the different wheels (Section 5), allowing us to reconcile these differences. This leads to the observation that the technical color wheels are broadly consistent with one another, provided we are precise about our specification of the principal colors in those spaces, but that the standard artist’s color wheel is substantially different from the technical color wheels (Section 6), because its primary and secondary colors are so strongly related to use of basic color terms in English.

Color has fascinated philosophers and artists since antiquity, but it is only in the last century that we have come to understand the psychophysical and biological mechanisms of color vision; so early writers could be said to be working in the dark. Aristotle described seven principal colors (white, yellow, red, violet, green, dark blue, and black) which he considered all to be mixes of white and black [39], a misconception that started to be challenged in the fifteenth century [1] but still held some sway until the eighteenth century. The discovery that red, yellow, and blue are the artist’s primaries was made in the early seventeenth century. Shapiro cites Parkhurst and Gage as reporting that four scholars independently discovered the artist’s tri-chromatic primaries. All four scholars were conversant with both art and the natural sciences, giving them access to the understandings needed to make this discovery. Shapiro asserts that it “… was the most important discovery in color before Newton’s own theory” [39, p. 624].

In the late seventeenth century, Newton conducted extensive investigations into the nature of color, discovering that white light split into an infinite range of colors: the visual spectrum. This discovery was at odds with the widely held belief that white was “pure” and could not be split and also at odds with the three primary colors discovered earlier that century, discrepancies that caused him much trouble to attempt to reconcile and which led to substantial challenges in his work being accepted. Nevertheless, in his writings before Opticks, whenever he listed his principal colors of the spectrum, he always added some phrase such as “with their innumerable intermediate gradations” to indicate that there were countless discernible colors, but in Opticks he omits to say this in all but one place, possibly in an attempt to placate his critics [39, p. 619]. Newton’s early work described five principal colors: red, yellow, green, blue, and purple, but he later added orange and indigo, leading to English’s current seven-color rainbow (see a longer discussion in the Appendix).

At the start of the nineteenth century, Goethe launched a challenge on Newton’s purely physical approach, tackling color instead as a perceptual phenomenon. To a technically trained modern, some of Goethe’s arguments can seem misguided when compared with Newton’s empiricism. But Newton was, in his own way, blinkered: fitting the data to suit his hypothesis rather than the other way round [35, 36]. The challenge Newton faced was that his evidence was inconsistent, because he was assuming that mixing lights (additive color mixing) and mixing pigments (subtractive color mixing) should produce consistent results. It was only in 1852 that Helmholtz deduced that different rules apply to the mixing of pigments and of lights [39].

The chemist, Chevreul, dyemaster at the Gobelin work in Paris, published De la Loi du Contraste Simultané des Couleurs et de l’Assortiment des Objets Coloris in 1839 [23]. This, and other emerging color theories, had substantial influence on artists in the nineteenth and early twentieth centuries. Itten says that “Delacroix… is the founder of the tendency, among modern artists, to construct works upon logical, objective color principles, so achieving a heightened degree of order and truth.” [13, p. 418]. The impressionists and post-impressionists, in particular, used theories of color contrast and optical color mixing.

Several early commentators on color, including da Vinci, noted that there appear to be four fundamental colors: red, yellow, green, and blue, in addition to black and white [20]. Hering formalized this into the opponent theory of color vision [9, 22]. Hering’s theory was further developed by Hård, Sivik, and Tonnquist in their creation of the Natural Color System (NCS). There is evidence that the four opponent principal colors are physiologically determined [16, 19]. Hering’s theory was not widely embraced at the time because there was no understanding of how responses to two different colors of light could interact to create a color-opponent signal. We know today that the neurons in the retina process the outputs of the light-sensitive cones to produce three channels of data to the brain: a high-resolution luminance channel, a lower-resolution red-green channel, and an even lower-resolution blue-yellow channel [4, p. 16], [22]. The opponent-color channels explain well several features of human vision, including the way in which color blindness manifests and the complementary afterimages caused after fixating on a colored field. Consistent with this theory is that you cannot perceive a color as having simultaneous components from either end of an axis, so a yellowish-green and a bluish-green both make sense, but a human can never perceive a color that is “reddish-green,” such a mixture being a nonsense.

Over far more than a century, philosophers, scientists, and artists have grappled with ways to represent and understand color, leading to many systems of color representation. Basic introductions can be found in computer graphics and design texts [12, Ch. 13], [24], [40, Ch. 20–22] [41], with more detailed explanations in specialist texts [4, 5, 7, 26], and a full history of color spaces in Kuehni and Schwartz’s 2008 book [28].

A color space is a three-dimensional representation of color. We can restrict ourselves to three dimensions because the human visual system has three types of receptor for color vision. All of the color spaces are mathematical transformations of one another. Hunter gives a detailed history of nineteen color spaces developed in the attempt to create a perceptually uniform space, starting with the CIE 1931 color space and Munsell’s original system, through to the CIELUV and CIELAB systems of 1976 [21, Ch. 8]. Derefeldt gives the background of the most important color appearance systems, including Munsell, NCS, CIELAB, and CIELUV. She gives their basic attributes, and the principles for scaling and notation of the variables. In particular, she makes a comparison of the hue spacing of the different spaces [9]. Note that there is considerable evidence that color vision is non-Euclidean, so any color space is not going to be a metric space, perceptually [4, p. 64]. For example, the CIELAB system has a cube-root relationship with the signals that are received by the cones in the human eye. This is to better match the perceptual response of the human visual system but means that linear mixes in the CIELAB system do not necessarily match mixes of pigments.

A color wheel is a representation of one dimension of a color space: hue. Color wheels have been used for centuries. The earliest known drawing of a color wheel dates from 1611 [34], a century before Newton’s Opticks [32].

A color wheel or, more accurately, a hue wheel, is a circle that passes through all of the spectral colors and then through the purples to join the two ends of the spectrum (Fig. 1). Hue is explicitly one of the three dimensions in some color systems, including NCS (Fig. 3) and Munsell (Fig. 4), and is implicit in others, where hue is a function of two or three of the principal dimensions of the space. For example, in the case of CIELAB, h∘=tan−1(b∗∕a∗). When considering a color wheel, the hues always appear in the same order around the wheel but they differ in which hues appear opposite each other and in the relative angular separation of pairs of hues.

A student may make an assumption that a “true” color wheel exists and that the different color wheels essentially stretch or contract sections of the “true” wheel to fit their predilections, as if the colors were painted on a rubber bicycle wheel and we nailed certain hues to certain points on the rim. The stretching and contracting is epitomized in the differences in the angles red-yellow and green-blue, shown in Table I. When a color wheel is used as a mechanism to describe hue, then such stretching or contracting is fair: the wheel is not purporting to show precise physical relationships. However, when a color wheel is used to describe relationships or mixes between distant hues, such as in defining the “opposite” of a hue or “harmonious color combinations” (Fig. 2), then this stretching and contracting becomes questionable.

Berlin and Kay proposed the theory that there are basic color terms in all languages [3]. These are the terms that you teach small children and which produce categories of color that are irreducible, that is, all other color terms are considered, by most speakers of the language, to be variations on these basic color terms.

In antiquity, classical scholars certainly privileged certain colors above others. In the distant past, the fundamental colors appear to have been severely limited. Berlin and Kay quote Geiger as suggesting that “Democritus and the Pythagoreans [fifth century BC] assumed four fundamental colors, black, white, red and yellow” [3, p. 136]. Elsewhere, Geiger comments that Aristotle [fourth century BC] “in his ‘Meteorology’ calls [the rainbow] tri-colored, viz., red, yellow, and green” [14, p. 57]. By the fifteenth century, things had developed a little further. Alberti cites three fundamental colors: red, green, and blue, combined with gray [1, Bool 1, paragraph 9] while da Vinci lists what we now call the color-opponent set of principal colors: red, yellow, green, and blue [20]. In the seventeenth century, Boyle listed the standard artist’s primaries: red, yellow, and blue [20], but added green and purple when actually conducting his experiments on color [6, p. 187]. In the early eighteenth century, Newton started with these five principal colors: red, yellow, green, blue, and purple, then added orange and indigo (see a longer discussion in the Appendix).

There is a question of nature versus nurture: how much the color categories are inherent in our psychophysiology and how much they are cultural constructs. There is good evidence that black, white, yellow, red, blue, and green are strongly tied to the perceptual mechanisms in the human brain [19]. Hardin notes that the four principal colors (yellow, red, blue, green) “… prove to be both necessary and sufficient for an English speaker to describe any spectral stimulus” [18]. The other basic color categories may be more culturally determined. Children are able to match and discriminate colors long before they have consistently codified the boundaries in color space of the basic color terms, so providing evidence that the boundaries are a social construct [2]. In any case, in order to communicate clearly between members of a language group, the learned categories must be at least partly a social construct, reinforced by parents, kindergartens, and primary schools because all members of the language group broadly agree on them.

Berlin and Kay identified that the number of basic color terms range between two (representing light and dark colors) and twelve, depending on the language. In English there are eleven basic color terms: red, orange, yellow, green, blue, purple, pink, brown, black, gray, and white. As an example of the irreducibility of these basic terms, consider how difficult it is to convince a child that brown is really “dark orange” or that pink is “light red” [18, p. 210]. You may teach a particular child or student to make finer distinctions, as between “cyan,” “azure,” “indigo,” and “turquoise,” but there is a cultural push toward teaching and agreeing on the eleven basic color terms [26, Ch. 11], and there is demonstrated effect of these basic categories on the ability to perform color discrimination [43]. The maximum number of basic color terms in any language appears to be twelve. Russian, and a few other languages, distinguish light blue (Russian goluboy) from dark blue (Russian siniy) [33]. This paper considers the case of English though most other European languages use the same eleven categories, which is important to our discussion because Itten, in particular, was working in German.

Rather than conducting new perceptual experiments, we are able to make use of results from three previous studies [3, 37, 38], which used color chips evenly chosen from Munsell’s color space.

Ignoring the monochrome black, gray, and white, there are eight basic color terms in English. Roberson et al. [37] experimented with an array of 160 colored chips, evenly spaced within the Munsell color system, asking English speaking subjects to categorize each chip into one of the eight color categories.

Figure 5 shows the mean color chosen by subjects for each color chip. In addition, each color region contains a small cross that marks the “best-example choice” for each of the eight colors, as described by Rosch [38]. Notice the difference in sizes of the different color terms: orange (5.5 cells), yellow (6.5 cells), and brown (9 cells), each take up only a small part of the color space compared with green (52.5 cells) and blue (36 cells). While I acknowledge that Munsell’s color space is non-uniform and is somewhat compressed in the yellow-red area and expanded in the blue-green area, that cannot explain the full magnitude of this difference. Over 50% of the chart is categorized as one of two terms blue and green; by contrast, red, orange, and yellow between them take up just 14% of the chart (see also Hardin’s comments on the relatively small sizes of the “warm” colors’ regions compared with the relatively large sizes of the “cool” colors’ regions [19]). Describing a color as “red,” “orange,” or “yellow” will always give a color close to the “best-example choice,” that is, the color will be close to what an average person would imagine it to be. By contrast, describing a color as “green” or “blue” can give a color that is a significant distance from the “best-example choice.” Hardin discuses the consistency of such studies, noting that, across the many studies, “No matter how many basic color terms languages might have, their foci [“best-example choices”] tend to cluster reliably in relatively narrow regions of the [Munsell] array, whereas boundaries are drawn unreliably, with low consistency and consensus for any language.” [18, p. 208]. As evidence that there is consistency between observers, consider that the NCS color system is predicated on there being good agreement between observers on what Berlin and Kay call the “best, most-typical example” of the four principal colors red, yellow, green, and blue [17].

Berlin and Kay undertook a different experiment [3], using a 320-Munsell chip array, in which, for each basic color term, they asked English speaking subjects to select all those chips that they would, under any conditions, categorize as being of that color. Figure 6 shows the regions in which they got an unequivocal response from their subjects. One important result, for our investigation, from Berlin and Kay’s work is that both cyan and indigo were unequivocally described as “blue” by their subjects.

Note that pink and brown do not appear on the standard color wheel. Pink is a light variant of red. Brown is a dark variant of orange. The basic English color terms along the visual spectrum are thus red, orange, yellow, green, blue, and purple. Of these, orange is a relatively recent addition to the basic color terms in English. Red (Old English réod), yellow (geolu), green (grene), and blue (blaw) are all ancient color terms. Purple was brought into English, from the Latin, in the ninth century. Orange, by contrast, was adopted only in the early sixteenth century. Its first attested use as a color name was in 1512. Prior to this it had been known as yellow-red (Old English geoluréod). It is unclear when orange became a basic color term in English, but it is a possibility that Newton’s description of the spectrum was an influence. Similarly, in German, gelb, rot, blau, and grün are ancient terms with words for orange and purple being more recent [25].

Figure 7 illustrates the principal colors of five color wheels in common use. It is immediately obvious that they do not map linearly to one another. The color that is diametrically opposite to blue ranges from yellow (Fig. 7(a),(c)) through orange (Fig. 7(b),(d)) to a red-orange (Fig. 7(e), but see also the Appendix). I briefly describe each of the five color spaces, including the purposes for which it was designed and the principal colors it uses.

Consider the structure of the various color wheels as a student would view them. Fig. 7(a)–(d) shows the result if you place the principal colors evenly spaced around the wheel, as they are in all diagrams in the student’s text books (e.g., Figs. 1, 3, 4). Table I tabulates the angle a student would measure between red and yellow and that between green and blue. The red-yellow angle varies from 60∘ (Fig. 7(c)) to 120∘ (Fig. 7(d)), while the green-blue angle varies from 120∘ (Fig. 7(c)) to 60∘ (Fig. 7(d)). In a non-metric space, these angles are, at best, approximate, but a student will still worry about why the angular distances around the wheels differ so markedly, especially if they have been trained to build the harmonious color combinations of Fig. 2, which explicitly require consideration of angle. They will also be concerned to understand why diametrically opposite color pairs differ between color spaces.

In theory, the artist’s RYB and the printer’s CMY color wheels should be identical. Both purport to have primaries that cannot be made by mixing other colors. Both purport to be able to create all hues from the three primaries. Both purport to have diametrically opposite colors that mix to make gray. However, the CMY color wheel is demonstrably the correct way to do this, given that these are the colors used in the vast majority of commercial color printing processes. The underlying theory is that each of the primaries theoretically absorbs exactly one-third of the visual spectrum. Berns [4, Ch. 6], for example, suggests splitting the spectrum into thirds at 500 nm and 600 nm. A theoretical cyan ink absorbs all red and orange light. A theoretical magenta ink absorbs all yellow and green light. A theoretical yellow ink absorbs all blue and violet light. Combinations of these three primaries can produce any hue. In practice, the spectra of the three inks are not perfect squares [4, pp. 154–5], [27, Figs. 1.4-20,-22], so the range of colors achievable is not as broad as would be possible with perfect theoretical primaries, but we can manufacture inks of sufficient quality to satisfy the vast majority of our printing needs.

The dramatic difference between the theoretically correct CMY color wheel and the artist’s RYB color wheel can be explained by considering the mechanism by which the artist’s colors are chosen. In Itten’s seminal writing on the color wheel [23], he writes: “… a person with normal vision can identify a red that is neither bluish, nor yellowish; a yellow that is neither greenish, nor reddish; and a blue that is neither greenish, nor reddish… The primary colors must be defined with the greatest possible accuracy.” There is no freedom here to allow red to be magenta, because magenta is a red that is distinctly bluish, nor is there freedom to allow blue to be cyan, because cyan is a blue that is distinctly greenish. I hypothesize that Itten is placing his three primaries at or near the “best, most-typical locations” in the Berlin–Kay sense. Itten then mixes his secondaries, which are all also Berlin–Kay basic color terms. So while Itten says that his hues are “… evenly spaced with complementary colors diametrically opposite each other,” his even spacing is in a linguistic sense rather than a physical one.

Note also how Itten defines his red, yellow, and blue. Red is “neither bluish, nor yellowish,” defined relative to the other two primaries. But yellow and blue are “neither greenish, nor reddish” (emphasis mine), so Itten’s three primaries are defined relative to the four principal colors of opponent-color theory [7, Sec. 11.3]. The NCS color system defines its principal colors in exactly the same way [17, p. 181], but uses this defining mechanism to create four principal colors rather than the three primary colors of the artist’s color wheel (Fig. 8(f) cf. 8(d)).

By having red and yellow as primaries in the artist’s color wheel, orange becomes a natural secondary and, because it is a basic color term, it is possible to mix orange so that, to the artist’s eye, it is neither too reddish or too yellowish, thereby occupying its “best most-typical” position in color space. With blue in the third of the primary positions, it is obvious from the English Berlin and Kay chart (Fig. 5) that the other two secondaries are going to be green and purple, if we wish them to also be basic color terms. As with orange, it is possible to mix these, as Itten says we should, so that they are well-balanced and not leaning toward the color on either side, which I hypothesize places them at the “best most-typical” positions.

The substantial difference between the artist’s color wheel and the other color wheels would not be a problem if the artist’s color wheel, as designed by Itten, were not so pervasive in education about color.

The challenge with creating the artist’s color wheel is that Itten had two aims that cannot be satisfied simultaneously: he wants his diametrically opposed colors to be perfect subtractive complementaries [23, p. 20] and he wants his primary and secondary colors to be mixed “very carefully” so that, perceptually, they do not “lean toward” either color on either side [23, p. 29]. The former is achieved correctly by the printer’s color wheel; the latter pushes the primary and secondary colors to their “best, most-typical locations” in perceived color space. The fact that the printer’s color wheel and the artist’s color wheel are substantially different demonstrates that these two aims cannot both be satisfied in the same color wheel (It is possible to create narrowband pigments for opposing colors in Itten’s scheme, so that the opposing colors mix to gray, but broadband pigments for the primaries do not allow coverage of as large a gamut as CMY.).

The artist’s RYB is thus an approximation to CMY, yet the artist’s color wheel remains by far the most popular color wheel, outside the technical sphere. This, in spite of the fact that the printer’s wheel is technically superior for mixing the widest possible range of colors. We must ask why it is that a color wheel that appears technically inferior should be so tenaciously held. I hypothesize that the artist’s color wheel’s success is owing to its use of the six basic color terms that correspond to spectral colors: if you are teaching color theory to children, you will gravitate toward using the color names with which they are most familiar.

One of the challenges in teaching technical printing is to explain to the student the special role of magenta and cyan, and to describe what they are in terms of the basic color terms (“reddish-pink” and “greenish-blue,” respectively). For example, Gleeson, in her text on the illustration of picture books, identifies magenta with red and cyan with blue [15, p. 53], while Cianciolo, writing on the same topic, twice mentions the four process colors, naming them as red, yellow, blue, and black [8, pp. 61, 88]. This is not necessarily a misunderstanding on the author’s part but a need to explain the technical concepts (“magenta” and “cyan”) in a language that is accessible to the general reader (“red” and “blue”).

As a framework for teaching color, RYB does admit the possibility of using colors other than the “best most-typical” and artists over the centuries have used a range of different reds, yellows, and blues as their primaries. However, magenta is outside the red zone in the Berlin and Kay diagrams. I hypothesize that the untrained observer has a challenge with accepting magenta as a primary, because it is not intuitively satisfying. Though magenta is the correct color for printing, it does not fall at one of the optimal points in Berlin and Kay’s diagram, being somewhere between red, pink, and purple. Red is much more satisfying, being one of the key colors in Hering’s theory of perception. Yellow, by contrast, is both a primary and an optimal point in linguistic color space, so we have no trouble accepting it. Cyan is a blue but it is not the most typical blue.

There is a further gloss on the use of RYB. Despite it being taught to children as a way of “mixing colors,” it would be extremely unusual for a professional artist to have just three colors on their palette. Rather, the artist’s color wheel is used as a framework within which to understand color relationships. This is because it is not possible to achieve all colors by mixing just a red, yellow, and blue; and because having a pure hue allows for consistency of color not achievable in repeated mixings. For example, Matisse used a palette of 17 colors, van Gogh 9 colors, and Fryer shows an example of his own work with a palette of 14 colors [13, p. 419]. And, while the color harmonies of Fig. 2 are widely taught, any professional artist or designer will use their own judgment of harmony rather than slavishly depend on this basic framework. Indeed, Itten himself says that different students find different combinations harmonious so that there cannot be a general principle that appeals to all [23, p. 23].

Some of the apparent differences between the color wheels can be explained linguistically. The most obvious example is that we recognize that “blue” has a broad spectrum of meanings and that the “best, most-typical” blue is an imprecise approximation to the true color represented by the word. In the printer’s color space, “blue” is an indigo, in Munsell’s color space it is tending to cyan, in the NCS and artist’s color spaces it is sitting at the “best most-typical” position, and in the traditional rainbow, ROYGBIV, it transpires that “blue” was originally used by Newton to mean “cyan.” But all of these are described informally by the single term “blue.” Likewise “red” and “green” are used in some color spaces to refer to colors that are not the “best most-typical” example of the color.

What can educators conclude from this? Using commonly understood terms, like “opposite” or “blue”, to also have a specific technical meaning leads to problems, unless one is careful to define those terms to have precise meaning. When educating students about color, we need to be careful to be precise in what we mean when we use terms like “blue.” In the printing industry, we already have this precision when talking about the CMY space, because cyan and magenta are not basic color terms, so our students understand them to have precise meanings, and yellow is a precise term in common usage, because it occupies such a small part of the overall color space (Fig. 5). But terms like red, green, blue, and purple all have imprecise meaning in English and we must be careful to ensure that we are defining them appropriately.

There is a remaining challenge, which is that the artist’s color wheel is at such odds with all of the other color wheels and yet is the first color system that most people will meet. I hypothesize that one reason for its tenacity is that it is a convenient approximation that allows educators to use six of the basic English color terms in explaining how color works.