Objects in nature derive their color from colorants they process that absorb or subtract certain wavelengths of light while reflecting other wavelengths back to the viewer. For example, a red apple really has no color, it merely reflects the wavelengths of white light that cause us to see red and absorbs most of the other wavelengths. The viewer (or detector) can be the human eye, film in a camera or a light sensing instrument.
The human eye contains two basic types of light receptors, rods and cones. The rods are sensitive only to the presence of light, not color. The cones are sensitive to color. During normal daytime vision, it is the cones, not the rods that actively contribute to visition. At night, the more sensitive rods take over and give us “night vision.” There are three groups of cones, each sensitive to a portion of the visible color spectrum – red light, green light and blue light. The brain receives signals from the cones, processes them, then evokes the sensation of color. Various combinations of light waves evoke the sensation of other colors.
Color perception varies form person to person. Perception is a subjective phenomenon influenced by many variables including the light source, surrounding colors, mood of the viewer and the individual variations in our visual systems. A small number of people have color-deficient vision. The mos common form is the inability to distinguish between reds and greens. These people are considered color-blind. This phenomenon may result from one type of cone missing or a defect that affects analysis in the brain. Color blindness affects 8% of men and less than 0.5% of women.