The use of color in cartography is extensive and subtle.
Choropleth maps use ‘color schemes’ or ‘color ramps’ - sets of colors that are used to represent each value on the map. These combinations of colors are also used for displaying raster images of other data, like elevation. There are roughly three types of color schemes: sequential, qualitiative, and divergent: Cynthia Brewer summarizes them well in this article.
Choosing good colors for maps is very difficult: colors need to be perceptually accurate, so that the gap between each color’s saturation and lightness looks consistent across the range. They also need to be safe for colorblind people, which includes 7-10% of men. Finally, colors need to fit expectations - for pre-existing associations, like green being fertile or red meaning warm, the color ramp shouldn’t reverse or misuse existing values.
Technology
ColorBrewer, a long-standing project by Cynthia Brewer, provides a set of ready-made, tested color combinations that you can use in your own maps.
To create a new color scheme from scratch, HCL-based color pickers help to avoid confusing color schemes. The Ames Color Tool is also useful for choosing perceptually correct scales.
If you don’t have colorblindness, you can use tools like Color Oracle to simulate its effect when designing maps.
Reading
- The Subtleties of Color, a series about choosing colors for maps and visualizations.
- Using Color in Information Display Graphics