Introduction
Color is a sensation that is aroused when light falls on the retina of the eye. Light may be perceived either as originating directly from a light source or as reflected light. White light is perceived as colorless because the eye is completely attuned to the characteristics of such light, and only a neutral color sensation is aroused by it. Color perception depends on the different degrees to which various wavelengths of light stimulate the eye.

Spectral Colors
White light can be dispersed by a prism, which resolves a beam of white light into its colored components, the spectrum. Visible light is electromagnetic radiation within a wavelength range of about 410 nm (nanometers) to about 770 nm. The various spectral colors may be characterized by their wavelengths within this range.

An object that reflects only the part of white light between 540 nm and 600 nm will appear yellow. Yellow light may also be generated by combining green and orange-red light (the colors adjacent to yellow in the spectrum) or by combining all colors except blue. Blue is called the complementary color of yellow; the other colors also have complements. Colored light mixed with light of its complementary color appears white.

The actual color sensation produced by an object is determined by a combination of the composition of the incident light and the object's reflective properties. An object illuminated by blue light can, of course, reflect only blue light. The color of the object will then be observed only as shades of blue or black. For example, yellow and orange objects reflect almost no blue light and, under these circumstances, will appear black.

Definition and Classification
Color has so many meanings for different observers that a strict definition is difficult, if not impossible. The chemist is conscious of color as a quality concerning a pigment or a dye; the psychologist describes color in terms of visual perception; and the physicist may define color in terms of qualities such as the wavelength of light and its intensity.

A description of color has its foundations in attempts to classify colors. The basic distinction is made between those colors with hue and those without it. The members of the first group—red, orange, yellow,green, blue, and so on—are termed chromatic colors; those of the second group—black, gray, and white—are called achromatic colors.

The next classification divides the chromatic colors into groups by hue,that is, all reds are together, all blues are together, and so on. In doing so a continuous circle of overlapping hues is formed, ranging from red through orange to yellow, and then through green to blue and violet. Violet overlaps red, thus completing the circle.

Achromatic colors are arranged in a single series from black through the grays to white. Some of the chromatic colors of a single hue are darker or lighter than others, and it is possible to match each degree of lightness to gray of the achromatic colors. This classification is known as brightness, or luminance.

If a particularly vivid hue is mixed with an achromatic color of the same brightness, the resulting stimulus depends on the relative amounts of these two components. This characteristic of color is called saturation. The achromatic colors have zero saturation; the saturation of chromatic colors has a value between zero and one.

All the colors can be classified to form a color tree by placing colors of the same brightness on a disk, with the hues placed consecutively around the disk and with the saturation increasing outwardly from the center. Similar disks of different degrees of brightness are placed in order of their brightness above and below. In this manner a color solid is evolved.

Primary Colors
The human eye is not a selective instrument; it cannot distinguish two superimposed colors as such. Taking advantage of this fact, in 1801 Thomas Young, and later Hermann von Helmholtz, found that it was possible to match any given colored light using a combination of three primary light sources. Occasionally one color was found that could not be matched by direct addition of the three primaries. It was always found, however, that if one of the primaries was added to the given color, the other two primaries could produce a color match with the combination of the sample and the third primary. The amounts of the three primaries required to produce a given spectrum color as a function of wavelength are called tristimulus values. In determining these values, the use of negative values of the primary are occasionally required.