This post defines and illustrates the various types of color combinations used in color theory. In order to focus just on the color schemes, the map examples are presented as slices of maps. For most of these examples, clicking on the map will take you to the full, original, source map.
MONOCHROMATIC color schemes have one or more shades of a single color. In mapping, we commonly see monochromatic color schemes representing the magnitude of a variable in a choropleth. In choropleth maps, remember to keep the number of shades in a monochromatic color scheme at five or less. More than five shades of the same hue are too difficult to track back to the legend.
Conversely, in a heat map, where the exact magnitude does not need to be tracked back to the legend, more shades could be used for a smoother appearance. (Both heat maps and choropleth maps can be non-monochromatic as well.) You might also see a monochromatic color scheme described as “sequential”.
ANALOGOUS color schemes contain colors that are adjacent to each other on the color wheel. These colors often create a subdued feeling for the finished map. Colors schemes with green and blue, for example, are ubiquitous in reference map design and, if done right, are calming to the eye.
COMPLEMENTARY color schemes have opposing colors. This is usually thought of in terms of the classic color wheel: colors on opposite sides are complementary. However, the exact hues can vary somewhat depending on the color model that you are using. The traditional complementary color pairs are red and green, blue and orange, and yellow and violet.
In mapping, complementary colors are often used in choropleth and heat maps. These are sometimes referred to as “diverging” color schemes when they are also accompanied by a hue progression from light to dark or from dark to light. In reference mapping, complementary colors are employed in order to provide sufficient contrast. For example, a map could have a green background and red reference lines.
NEUTRAL color schemes are only composed of black, gray, and white. Sometimes this definition is expanded to include colors that are particularly unsaturated, or absent of hue, such as beige and taupe. In cartography, neutral color schemes are used in report maps or other publications that will be printed on black and white printers. However, they are not unheard of in webmap design.
POLYCHROME color schemes have colors from all over the color wheel. Maps with polychrome schemes are common on maps where multiple layers are visible at the same time, as the use of many colors is sometimes the only way to differentiate the layers adequately. This schema is also used on maps with a single layer with multiple feature-type differentiation needs.
These schemes need to be visually cohesive despite their many and varied hues. Map makers may use color schemes that are already proven, by using reference materials such as paintings, books (Cartographer’s Toolkit contains 30 pre-formed and pre-tested map palettes, for example), and the like, to find a good palette. Applying the stronger colors to smaller features is one method of achieving color harmony in such a scheme. The reverse—strong colors throughout with a few, smaller, bright colors—is also sometimes effective.