Choropleth maps fill regions with color. A color gradient and density distinguishes regions from one another. Use to compare different regions such as continents, countries, states, territories, zip codes, or census tracks. Provide a legend. Keep the gradient of colors within a limited range. This will allow the reader to easily compare the regions.
Sosulski, Kristen. Data Visualization Made Simple (p. 53). Taylor and Francis. Kindle Edition.