The trichromatic theory, also referred to as the Young-Helmholtz Trichromatic Theory, is a theory on how humans perceive colors. The theory suggests that the human retina has three receptors for color. Each receptor is sensitive to a different color. One receptor is sensitive to green, one to blue and another to red.
If this theory is correct, the human retina has no receptors to see colors other than green, blue and red. To make the brain perceive colors like orange and brown, the receptors have to be stimulated in the right way to produce the desired color. The pigment in the receptors is sensitive to certain wavelengths. A pigment is more likely to react on a cellular level if a photon that it's most sensitive to hits it with a specific wavelength, producing various colors.
The trichromatic theory originated in the 18th century, when Thomas Young suggested that the ability of the human eye to see different colors was the result of only three different photoreceptor cells. This theory was later expanded by Hermann von Helmholtz, after his experiments showed that people needed three wavelengths for the brain to create a standard range of color. It wasn't until 1956 that the trichromatic theory received physiological backing from Gunnar Svaetichin.