1758: ROY-G-BIV and Newton Oct 7, 2019

The reason that the rainbow has 7 colors is because of Isaac Newton. While other color-schemes had been devised since antiquity, the mnemonic ROY-G-BIV—invented by Newton—was made to describe the spectrum of light. It originally only included 5 colors: red, yellow, green, blue, and purple. In addition to RYGBV making for a worse mnemonic, Newton added 'orange' and 'indigo' to make the number 7—though he could have chosen any number—in order to correspond with the number of notes in a musical scale. This is also why purple was divided into indigo and violet. Today we call Newton's blue 'cyan' and would likely call his 'indigo' 'blue', but the terms are kept due to his mnemonic.