The Datawrapper blog has been publishing some data visualization posts. This article gives a an overview of ways to make your legend colors more accessible to viewers:

You designed a useful (and fun!) color key for your data visualization to explain to your readers what your colors mean? Great! Now you’ll want to make sure people don’t forget those associations between colors and categories. This article explores a few ways to do so:

  1. Keep your color key always close by
  2. Reuse your colors in annotations
  3. Make your color key interactive
  4. Reuse your colors in tooltips
  5. Reuse your colors in text around your visualization

Better use of color keys