Young and Helmholtz, in the mid 1800’s, came up with the first theory of color vision– trichromatic theory.  This theory holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths…one for red, one for green, and one for blue.  All colors can be seen, according to this theory, because of color mixing.
	But what about yellow? Is it just reddish-green?  Edward Hering, in 1878, proposed opponent process theory, which holds that color perception depends on receptors that make antagonistic responses to three pairs of colors…red on,green off; yellow on,blue off; black on,white off.  This not only takes care of yellow, but also explains the phenomenon of complimentary afterimages.
	While researchers argued about which was right for almost a century, most psychologists now agree that it takes both theories to explain color vision.