Complementary colors are those that can be mixed to produce white light. If a line is drawn from the white point through a point representing a specific color, the intersection of this line with the spectral locus defines the dominant wavelength . If the line crosses the purple boundary, the dominant wavelength cannot be defined, and such color is called nonspectral . Indeed, there is no magenta in the rainbow. The chromaticity diagram is also used to define color gamuts , or color ranges, that show the effect of adding colors together.
Color gamuts are simple polygons positioned on the diagram.
Because the Center of Gravity Law of color mixture holds for the diagram, all colors that are additive mixtures of the vertices of a gamut, are necessarily located inside the gamut. The horse-shoe shape of the diagram explains why visible red, green and blue cannot be mixed additively to match all colors. The diagram is thus useful to compare the gamuts of various color monitors, printers, slide films, and other hardcopy devices. Since usually the gamut of a printer is smaller than the one of a display, a reduced gamut of colors should be used with the display to achieve accurate reproduction.