The Greek epic poet Homer believed that rainbows were made of a single color, purple. (How decidedly unpoetic).
The Greek philosopher Xenophanes elaborated by bestowing the rainbow with another two colors, saying that it was comprised of purple, yellow‐green, and red.
Aristotle agreed with Xenophanes in his treatise, Meteorologica: "The rainbow has three colors, and these three, and no others". Apparently this was a hot topic!