In the CIE1931 xy chromaticity diagram, the set of visible colors is delimited by the spectrum locus representing the monochromatic radiations and by the purple line, set of nonspectral colors obtained by binary mixing between monochromatic radiations at the two extremities of the
visible range. In contrast with the spectrum locus, which represents, for most wavelengths, bright and maximally saturated colors, the purple line rather represents colors at the perception limit, corresponding to spectral powers with very low luminous efficacy. The notion of “practical”
purple lines is discussed. It comes down to the definition of the blue and red monochromatic primaries of RGB light emitting systems. The choice of monochromatic primaries allowing the widest gamut while maintaining a good luminous efficacy over its whole perimeter is still an open question:
by choosing red and blue primaries further in the extremes of the visible spectrum, the gamut size is increased but the luminous efficacy of the primaries, thereby of all reproducible colors, is decreased. As optimal trade-offs, we suggest two RGB systems favoring either the gamut size, or
the luminous efficacy.