Choropleth maps, the ones where regions are filled with colors based on data, grow easier to make. However, choosing colors, the number of colors, and the breakpoints is often less straightforward, because the answer is always context-specific. Lisa Charlotte Rost, now at Datawrapper, provides a rundown of the decision process.
The explanation is in the context of the Datawrapper tool, but you can easily apply the logic to your own workflow.