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.


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