Amplitude for color mixing is different from other amplitudes such as loudness. Color amplitude must refer to a light's ability to look different from other lights, to express its redness or other chromatic intensity, so that its color is not lost during transduction. To reveal independent stimulus dimensions, a set of orthonormalized color matching functions is derived, similar to opponent color primaries. Following an idea of Jozef B. Cohen, it is then assumed that a light of unit power varies in wavelength through the spectrum. The track of that light in the orthonormal color space gives a curve that Cohen called “the locus of unit monochromats,” after he found it by different steps. The locus defines a surface that is interesting but not complicated, which Cohen called “butterfly wings.” Projecting the locus into a plane normal to the achromatic axis gives a boomerang shape with 3 well-defined local extreme points. The extrema are William A. Thornton's Prime Colors, so a few steps reveal the inner workings from which the Prime Colors arise. The results can explain color mixing to beginners, but are also quantitative and ready for practical use.