The theory that there are three kinds of cones in the retina, each of which responds primarily to a specific range of wavelengths.
To understand why some people are color blind, we need to consider the basics of color vision. Two processes are involved. The first process is explained by the trichromatic theory of color vision. This theory suggests that there are three kinds of cones in the retina, each of which responds primarily to a specific range to wavelengths. One is most responsive to blue-violet colors, one to green and the third to yellow-red (Brown & Wald, 1964). According to trichromatic theory, perception of color is influenced by the relative strength with which each of the three kinds of cones is activated. If we see a blue sky, the blue-violet cones are primarily triggered, and the others show less activity.