Lisa Charlotte Rost of DataWrapper often writes about data visualization and lately she has focused on the (im)proper use of color in visualization. In this recent blog, she gives a bunch of great tips and best practices, some of which I copied below:

color in data vis advice
Gradient colors can be great to show a pattern but, for categorical data, it is often easier to highlight the most important values with colored bars, positions (like in a dot plot) or even areas. [https://blog.datawrapper.de/colors/]
color in data vis advice
If you need more than seven colors in a chart, consider using another chart type or to group categories together. [https://blog.datawrapper.de/colors/]
color in data vis advice
Consider using the same color for the same variables, but do differentiate between categories, even across graphics. [https://blog.datawrapper.de/colors/]
color in data vis advice
Using grey for less important elements in your chart makes your highlight colors (which should be reserved for your most important data points) stick out even more.  [https://blog.datawrapper.de/colors/]
color in data vis advice
Consider color-blind people. There are many different types of color blindness: Use an online tool or Datawrapper’s automatic colorblind-check. [https://blog.datawrapper.de/colors/]
 You can find additional useful tips in the original DataWrapper blog.