The Neural Control of Vision
J. The Processing of Color

Our ability to see color is brought about by three cone types, the red, green and blue, which have already been noted. Young in the early part of the 19th Century proposed that we could perceive all colors by having only three broadly tuned photoreceptors the relative signals from each which can provide accurately our color experiences. This is still one of the great computational models of all time much of which has been borne out subsequently. One problem with the model, as noted first by Herring, is that it does not explain color opponency. Opponent colors are red and green and blue and yellow. We do not experience the such colors as bluish yellow or reddish green. When you stare at a red spot on a white background, the afterimage is green, and when you stare at a blue spot the afterimage is yellow.