Color perception begins in the retina, the light-sensitive layer at the back of the eye. Two main theories explain how colors are seen: the trichromatic theory and the opponent-process theory. The trichromatic theory, proposed by Thomas Young in 1802 and extended by Hermann von Helmholtz in 1852, suggests that color vision is based on three types of cone receptors in the retina. These cones are sensitive to different but overlapping ranges of wavelengths corresponding to red, blue, and green. In color-matching experiments, participants adjust the intensities of three different wavelengths of light (usually red, green, and blue) to match a given target color. The fact that any color can be matched by combining these three colors supports the idea that human color vision relies on three types of cones in the retina, each sensitive to different but overlapping ranges of wavelengths. For instance, the perception of yellow can be achieved by activating red and green cones together.

Color blindness supports the trichromatic theory. Most color-blind individuals, who are predominantly men, can see some colors but not others. The type of color blindness depends on which cone type is defective or absent. For example, if the red cones are not working correctly, a person might have difficulty distinguishing between red and green.

In 1878, German physiologist Ewald Hering observed that some colors cannot be seen together while others can. For example, it is possible to imagine a greenish blue but not a reddish green. It was also noticed that the trichromatic theory could not fully explain afterimages, which are images that remain in vision after looking away from an object. For instance, after staring at something red, a green afterimage may appear. Similarly, after looking at something yellow for a while, a blue afterimage may appear. Trichromatic theory cannot explain these color pairs and the resulting afterimages.

The opponent-process theory, also proposed by Ewald Hering, explains how cells in the visual system process colors in opposing pairs: red-green and blue-yellow. According to this theory, certain cells are excited by one color and inhibited by its opposite. For example, a cell might be excited by red and inhibited by green or excited by yellow and inhibited by blue. This theory also explains afterimages. Staring at a red object for an extended period and then looking away can result in seeing a green afterimage. This occurs because the red-green visual system becomes fatigued and temporarily rebounds upon shifting focus.

The three types of cones in the retina are connected to retinal ganglion cells, which convert the three-color code into the opponent-process code. For example, a green cone might inhibit a particular ganglion cell, while a red cone excites it. This dual coding system allows the brain to interpret complex color information efficiently.