I thought this was a very helpful guide on using colour in figures. There are a few rules, but one comment from the whole document stands out. “If colour serves a purpose, but something other than colour would do the job better, avoid using it”.
Here are the simple rules:
1. If you want different objects of the same color in a table or graph to look
the same, make sure that the background—the color that surrounds
2. If you want objects in a table or graph to be easily seen, use a background
color that contrasts sufficiently with the object.
3. Use color only when needed to serve a particular communication goal.
4. Use different colors only when they correspond to differences of meaning
in the data.
5. Use soft, natural colors to display most information and bright and/or dark
colors to highlight information that requires greater attention.
6. When using color to encode a sequential range of quantitative values,
stick with a single hue (or a small set of closely related hues) and vary
intensity from pale colors for low values to increasingly darker and brighter
colors for high values.
7. Non-data components of tables and graphs should be displayed just
visibly enough to perform their role, but no more so, for excessive salience
could cause them to distract attention from the data.
8. To guarantee that most people who are colorblind can distinguish groups
of data that are color coded, avoid using a combination of red and green in
the same display.
9. Avoid using visual effects in graphs.