This is a nice article explaining why using a rainbow (red-orange-yellow-green-blue-indigo-violet) is a bad idea. The colors produce an artefactual banding pattern, they do not follow a consistent trend from light to dark, they cause trouble for people with color blindness, and they translate poorly to black-and-white reproductions. The article also shows some nice alternatives. Thanks to @EarlGlynn for sharing this.
Matt Hall. No more rainbows! Agile blog, 2017 (December 15). Available in html format.
You can find an earlier version of this page on my blog.