4.4 Cyan, magenta and yellow do better than red, yellow and blue
If a painter must choose three paints, something close to the printer’s three serves better than the painter’s: a yellow, a magenta-red and a green-blue in place of a yellow, a red and a blue. The reason is easiest to see with the visible spectrum cut into thirds, named after the light in them: blue, green and red. A yellow pigment absorbs the blue third and passes the other two. A magenta absorbs the green third, and a cyan the red third. Any two of these pass one third in common, and their mixture keeps what both pass. Yellow and magenta share the red third and make a red-orange; yellow and cyan share the green and make green; magenta and cyan share the blue and make a blue-violet. Printing inks are chosen for this reason [3].
A red pigment absorbs two thirds of the spectrum, the blue and the green, and passes only the red. A violet-blue absorbs the green and the red, with a little of the red leaking through, and passes the blue. A red and a violet-blue share nothing but that leak, and they mix to the gray of Figure 13. Red and yellow share the red third and mix well. Yellow and blue share a little green, where the edge of the yellow’s reflectance overlaps the tail of the blue’s, and they mix to dull greens. Of the three secondary colors, the mixtures of two primaries, the red-yellow-blue set makes one well, one dully and one not at all, while the cyan-magenta-yellow set makes all three.
With the calibrated paints the advantage is real, though smaller than the picture suggests. Hansa yellow medium, quinacridone magenta and phthalo blue cover about half as much again of the color plane as hansa yellow medium, pyrrole red and ultramarine, averaged over lightnesses from fairly dark to fairly light, and about twice as much in the dark colors (Figure 30 shows three of those lightnesses). The red-yellow-blue set wins in two narrow wedges, the scarlets and the violet-blue, which are its own red and blue. The magenta set wins across whole regions of the circle, the violets and the greens; at a violet near dioxazine purple’s hue and a fairly dark lightness, for one, it mixes colors with more than twice the chroma.
The real pigments are not the ideal blocks of the picture. Quinacridone magenta reflects some violet and is dark, phthalo blue only approaches a cyan and is dark too, and the printer’s inks are made transparent and thin, which paints are not. That is why the magenta set’s advantage is smaller than the blocks would suggest, and it loses the scarlets, which need a fourth paint of their own. The painters’ six-paint palette, which adds a scarlet and a violet-blue to the three, is a recognition of this, and Section 3.8 tested its rule.
Sources
[3] R. S. Berns, Billmeyer and Saltzman’s Principles of Color Technology, 4th ed., Wiley, 2019.