Sometimes, you might not want a full-color map, but rather a black-and-white or grayscale map with just a splash of color. You might be asking, "Why would I limit myself?" For print publications, costs can often be reduced by printing in black and white with a single added color, called a spot color. In addition, a tiny amount of accent color can really pop your data off the page. An early example is this cholera map of Exeter, compiled by Thomas Shapter in the 19th century.
The rather gruesome subject jumps off the map, with the only color being the red bars representing deaths: