Young-Helmholtz Trichromatic (three Color) Theory
Also known as the Trichromatic Theory, this is a theory of color and how humans perceive color. According to this theory, the human retina contains three different receptors for color (meaning each one is most sensitive to one color): one is most sensitive to red, one is most sensitive to green, and one is most sensitive to blue. These color receptors combine the colors to produce the perception of virtually any color. You notice that there are no receptors specific to orange, but by stimulating the right cones in the right way, orange color is produced.