Light's color is determined by the wavelength of the light energy striking the eye. The eye contains three kinds of cells that are specialized for picking up only certain wavelengths of light. When this information is relayed to the brain, the wavelength is interpreted as color.
The human eye contains cells, called cones, which are filled with one of three kinds of photopigments that selectively absorb only certain wavelengths of light. One is specialized for picking up short wavelengths, another for medium wavelengths and the third for long wavelengths. There is some overlap between the pigments' ranges, but none are able to see the full range of visible light on their own. When one of the pigments is not present or does not work properly, color blindness is the result.
In eyes with normal color vision, light that has a wavelength between 377 and 455 nanometers is perceived as blue, between 632 and 780 nanometers is perceived as red, and between 492 and 577 nanometers is perceived as green.