Seeing red…and yellow…and green…and: we owe our appreciation of color—what it is and how we perceive it—to scientists and artists. Do we also have some hungry primate ancestors to thank for the great pleasure it brings us?

Painters' experience with color mixing seemed at odds with Newton's claims. When mixed in equal parts, red, yellow, and blue paints yield a murky brown, not the white that Newton said would result from combining all the colors of the rainbow. This apparent inconsistency offered plentiful ammunition to Newton's detractors, Goethe among them. Any fool could see that no mixture of pigments produced pure white or anything like it.

James Clerk Maxwell dispelled the confusion in 1855 when he explained that mixing light rays of different wavelengths produces color in a different way than mixing pigments does: the former works by a kind of addition, the latter by subtraction. Maxwell showed that three kinds of light--orange-red, blue-violet, and green (a triad usually denoted simply as red, blue, and green)--suffice to generate almost any color. This additive method is how television screens, for instance, make color. On the other hand, making colors by mixing pigments is a form of subtractive mixing. A red pigment absorbs--that is, subtracts--the blue and green rays of light, and much of the yellow; only red light is reflected. A yellow pigment would remove the blue, violet, and some of the red and green. So mixing red and yellow narrows the range of unabsorbed rays, leaving just those in the orange part of the spectrum. Each time a pigment is added to a mixture, another chunk of the spectrum is subtracted from the reflected light, and the color gets muddier.

Goethe may have been unfair to Newton, but he was right to stress that color is not about light alone. There is also the matter of how we perceive it--and this is the trickiest business of all. Maxwell agreed, averring that "the science of colour must be regarded as essentially a mental science."

Perception is what happens when the eye and brain meet color and form. In 1802, English scientist Thomas Young proposed a theory of color vision based on the primaries red, blue, and green. He postulated that the retina of the eye contains "particles" that respond to rays of light by vibrating in resonance with them and that these vibrations create a signal that is dispatched along optic nerve to the brain. Young suggested that three types of particles, each sensitive to one of the three primaries, are enough to enable us to perceive a full range of colors. People who are color-blind, he proposed, lack one type of light receptor. Later in the nineteenth century, German scientist Hermann von Helmholtz developed Young's ideas further, and the resulting trichromatic (three-color) theory of vision bears both their names.

We now know that Young's particles are light-sensitive cells in the retina, of either a rodlike or a cone-like shape. Each human retina contains about 120 million rods and 5 million cones.

Experiments in the 1960s confirmed Young's 1802 hypothesis of trichromaticity by showing that cone cells come in three varieties, each with a different color sensitivity. Some respond most strongly to yellow light (as reflected from, say, a yellow flower), some to green, others to violet. The three types of cones are often, and somewhat misleadingly, equated with Maxwell's additive primaries of red, blue, and green. They are more accurately denoted as responsive to (that is, most strongly absorbing of), respectively, long (L), medium (M), and short (S) wavelengths of visible light. Together these three types of cone cells allow us to perceive all colors. A mixture of red and green rays, for example, can stimulate the L and M cone cells in the same ratio (about 70 L to 30 M) as does pure yellow light--and so the color sensation is identical in both cases. This is why the additive mixing of red and green produces yellow. The overall sensitivity of the eye to any particular color is the sum of the responses for all three types of cones. The neural signal increases steadily from red to yellow and then declines from yellow to violet, so yellow is perceived as the brightest color. The S (blue-violet) cones are the least sensitive of the three, which is why fully saturated blue looks relatively dark.