The sensation of color depends on the composition of light which is a mixture of white light and colored light. The colored light could have a dominant wavelength or hue. The hue that dominates is know as saturation (chroma). If the hue is diluted with white light, the saturation decreases.

There are three receptors in the eye that are activated by different wavelengths. This is why colors are charted by a mixture of three primary lights. Figure 1 is an illustration of James Clerk Maxwell's color triangle. The three apexes represent three primary colored lights: blue-violet, orange-red, and green. A multitude of colors can be produced by mixing lights of the three primary colors, but not all colors. A specific color (i.e., an unsaturated greenish blue) can be represented by a point on the triangular grid.

FIGURE 1. Maxwell's color triangle. The apices indicate the additive primary colors. The edges, the subtractive colors. Many but not all colors can be created by a mixture of the three color lights. The closer a point is to an apex the greater the proportion of the light is represented by that apex.

In order to represent all colors, three imaginary or "ideal" primaries must be used. The Commission Internationale de l'Eclairage (CIE) in 1931 defined the chromaticity curve with a standard observer and three ideal standard sources (modified in 1967). The chromaticity diagram is constructed by drawing a color triangle with three ideal, but non-existent, primary colors at each corner (See Fig. 2). The X-axis indicates the amount of ideal green that would be mixed with blue. The Y-axis indicates the amount of ideal red that would be mixed with blue. Values along the two axes represent a given color.

Superimposed on the triangle is the CIE chromaticity curve. It represents a band of pure spectral colors as a solid curve from violet up to green and down to red. The broken line connecting 380 nm and 700 nm are the non-spectral colors of purple which is obtained by mixing violet and red light beams. All the color which a human being can see are contained within the area bounded by the solid and broken lines. The central point at White in the diagram is the white produced by an equal mixture of the three primary colors.

FIGURE 2. CIE chromaticity diagram illustrating wavelengths in nanometers (nm) and energies in electron volts (eV). The area encompassed by the curved line and broken line segments include all visible colors. The pure spectral colors appear along the curved edge.

A mixture of two spectral lights can be represented as a point on the line joining the light point on the spectral curve. The broken line (Fig. 2) joins the blue light at 480 nm with yellow light at 580 nm. Following the broken line we go from spectral (saturated) blue to white to pale yellow to saturated yellow. A mixture of the correct amounts of 480 nm blue light and 580 nm yellow light forms any of the colors located in between. In the same way, purple colors can be formed by a mixture of red light with violet light as indicated on the broken line. A pair of colors that result in white (line joining the two colors passing through the white point) are called complementary colors. For example, blue light and yellow light form a complementary pair as do orange (600 nm) and blue-green (cyan, 488 nm). The mid-point (White) can be used as the origin to describe color as a mixture, in proportions of white light of a given wavelength. This wavelength, referred to as the dominant wavelength and the color associated with this dominate wavelength is termed the hue. The sensation of color is described in terms of hue. The amount of hue making up the composition of light is known as saturation (or chroma). The dominant wavelength points on the spectral curve are shown as a solid line in Figure 2 are fully saturated. As a dominate wavelength or hue is diluted with white light the saturation is decreased.

FIGURE 3. Illustrates different ways of obtaining metameric beams of pink light. Each graph indicates a mixture of white light to obtain pink light. A: shows a mixture of white and orange. B: a mixture of red with cyan. C: a mixture of red, green, and violet. To the human eye these metameric colors would all appear the same.

The following program shows how the computer uses the three primary colors Red, Green, and Blue to produces various shades of color.