How humans perceive color depends on the interplay of three elements: the nature of light, the reflective properties of an object, and the ways in which our retina and visual cortex process light waves. Regardless of what medium we work in--be it paint, print, or the Web--our ability to effectively use color depends on these processes.

A rainbow of color
In the late 17th century, Isaac Newton demonstrated that color did not reside in objects themselves but was rather a product of light, and that white light could be reconstructed by combining all of the individual wavelengths in the visible light spectrum. These wavelengths corresponded to seven individual colors: red, orange, yellow, green, blue, indigo, and violet.

The visible light spectrum isolated by Newton in his experiments makes up a small portion of the electromagnetic spectrum that ranges from low frequency, long wavelength regions (such as radio waves) to high frequency, short wavelength regions (such as X-rays). In between infrared and ultraviolet waves resides the visible light spectrum that includes the range of waves from 700nm (red) to 400nm (violet). While Newton proved that these individual wavelengths produced white light when combined, in actuality only three visible wavelengths are necessary to reconstitute white light: red, green, and blue.

Absorption and reflection
When these light waves strike an object in nature, the object can transmit, absorb, or reflect various individual light waves. Depending on the nature of the object and its atomic structure, it might reflect green light while absorbing the other wavelengths. The retina and visual cortex process this reflected light to produce our perception of color.

When artists and designers reproduce color on canvas or paper, they mimic this process by using pigments that absorb certain light waves while reflecting others. For instance, to produce green, we use pigments that absorb red and blue wavelengths. This process forms the basis of the color models used in both painting and print media.

The eyes have it
Of course, our ability to process light waves, whether reflected off of an object or emitted from a light source, depends on the retina of the human eye and the visual cortex of the brain. The retina contains three receptors, or cones, that respond to certain light wave frequencies. Red cones respond to low frequency waves, green cones respond to middle frequency waves, and blue cones respond to high frequency waves. These cones are not binary but rather act as channels that transmit levels of stimulation to the visual cortex, which processes the sum of these signals to produce the perceived color.

In order to produce the desired color, artists and designers must either add or subtract light waves, so that only certain light waves are reflected to the receptors. Whether you add or subtract light depends on the medium in which you are working.