The use of color on maps is much more than simply a way to construct a legend. Color carries visual meaning, it directs attention, it can emphasize or obscure features. Thus, color selection is an important element of map design. The human eye can distinguish tens of 1000s of different colors, but only 20-30 gray tones. Super VGA monitors can display up to 256 simultaneous colors from a total of several million possibilities.
More advanced monitors can display still more colors.
On a static map, however, the eye cannot relibly match more than 16-20 simple colors from the legend with the map. Still, there is a practical limit to how many colors should be displayed on a map. Maps are easiest to interpret when the number of colors does not exceed 10 to 12. Color may carry standardized meanings in particular applications. Such color schemes are often varied to suit the local circumstances.