A Rainbow is the the semi-circular arc that appears in the sky, usually after summer rain has fallen. There are reputedly seven bands of colour in it (Red, Orange, Yellow, Green, Blue, Indigo and Violet) but most people can only see six.

Sir Isaac Newton, who first explained how it was formed and studied it in depth, was, like 14% of the human population of Earth, colour blind, so he had to rely on the eyes of his assistant, which were equipped with seven-colour-perception, and were able therefore to discern a difference between indigo and violet.