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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 at https://agilescientific.com/blog/2017/12/14/no-more-rainbows.