Two theories outline the method by which we see color, the trichromatic theory and the opponent process theory. Each theory is quite different from the other, not simply a modification of one theory, with unique explanations as to how we see color.
The Trichromatic Theory of color was first proposed by Thomas Young, an 18th century English physician and physicist. He theorized that color vision was possible because the eye of humans, and other animals that see color, has three types of color receptors.
Hermann von Helmholtz, a German physician and physicist, showed that normal vision does require three wavelengths of light to create a normal range of colors. These three types of photosensitive cells each contain a pigment that is sensitive to a different wavelength of light, long (L) or red cones, medium (M) or green cones, short (S) or blue cones.
The underlying premise of the original theory is that it is possible to create all colors of the spectrum by mixing two primary colors.
Continued research has indeed shown that the center of the retina is closely packed with cones that are more receptive to varying wavelengths of light. They are not, however, only sensitive to one wavelength of light. How sensitive each type of cone cell is to each wavelength of light, very sensitive to barely sensitive, is information the brain uses to determine color.
The Opponent Process Theory of color was proposed by Evald Hering, in 1892. Previously, groundwork had been laid in "Theory of Colours" written by Johann Wolfgang von Goethe in 1810 and the construction of his color wheel of opposing colors.
This theory states that the cones are linked in such a way that they for three opposing pairs-blue/yellow, red/green, black/white. If one of the pair is activated, the action of the other is repressed. In addition, the members of the pairs are not in the same location so there are no colors such as bluish-yellow or reddish-green. This theory also explains why a green afterimage is seen after gazing at a red patch and then looking away to a white field.
This theory did not attract much attention until 1957 when visual scientists at Eastman Kodak, Leo Hurvich and Dorothea Jameson invented the hue cancellation method that could be used to evaluate opponent processing. Hue cancellation uses monochromatic lights and determines what hue is necessary to cancel the perceived or opposite hue.
While it has been popular to pit one theory against the other the latest evidence indicates both theories are needed to explain how we see color. The trichromatice theory explains the process early, at the receptor level, while the opponent process explains more of the processing done by the brain. In reality, color processing begins in the retina with mechanisms described by the trichromacy theory. Opponent theory describes how this information is processed so we can name the colors of the objects we see.