Any color you choose can be matched by a mixture of short, medium and long wavelength light (i.e., blue, green and red light). This perceptual observation led to the formulation, early in the 19th century, of a neurophysiological hypothesis: The eye contains three kinds of distinct color-sensitive receptors (cones); just as colors themselves can be composed of lights of different spectral character, so we can see the vast range of visible color thanks to the joint operation of only three distinct kinds of receptors.
This is a beautiful example of the primacy of experience in the study of the brain-basis of consciousness. Before you can even begin to think about how the brain enables us to see or feel or (more generally) experience what we do, you need to pay careful attention to what our experience is actually like.
And, so, it was further attention to the experience that led scientists to realize the shortcomings of what came to be known as the Trichromatic Theory of Color. Indeed, purple is just a reddish-blue or a bluish-red; you can actually see the red and blue in the purple, and you can imagine a purple becoming more and more blue until it is entirely blue.
The Trichromatic Theory tries to explain these phenomena by suggesting that we see purple when our red and blue sensitive cones (that is, our long wave- and short wave-sensitive cones) are activated at the same time. Different purples correspond to different ratios of activation.
But now, consider the case of yellow. You get yellow by mixing red and green light, just as you get purple by mixing red and blue. But yellow isn’t reddish-green or greenish-red in the way that purple is reddish-blue. In fact, there is no such thing as reddish-green. Moreover, you don’t see red or green in yellow the way you see blue and red in purple. Yellow, like blue and red, but not like purple, is unary, not binary.
The Trichromatic Theory has no resources to explain facts about color vision such as these. In order to explain them, neurophysiologists were led to propose a totally different kind of theory of neural processing beyond the retina (the so-called Opponent Processing Theory). read full story at npr