Abstract
For most of us, the phrase "all the colors of the rainbow" conjures an image of the natural rainbow1 as a paragon of color variety and vividness. Indeed, both our language and art often invoke the rainbow as a color palette without peer.2 Yet as a color standard, the rainbow has an oddly contentious history. For example, arguments about the number of rainbow colors date to antiquity, with observers as keen as Aristotle3 (who favored three colors) and Seneca the Younger4 (who favored an indefinite number) among the disputants. That this disagreement still persisted in Georgian England2 (and indeed to the present) hints that the rainbow poses special perceptual problems.