ROYGBIV is arbitrary Jan 29 2014
In a short video from The Atlantic, science writer Philip Ball explains why Isaac Newton picked ROYGBIV (red, orange, yellow, green, blue, indigo, and violet) for the colors of the spectrum and not 3 or 6 or even 16 other possible colors.
Newton was the first to demonstrate through his famous prism experiments that color is intrinsic to light. As part of those experiments, he also divvied up the spectrum in his own idiosyncratic way, giving us ROYGBIV. Why indigo? Why violet? We don't really know why Newton decided there were two distinct types of purple, but we do know he thought there should be seven fundamental colors.
Ball is the author of Bright Earth: Art and the Invention of Color, which looks pretty interesting. His mention in the video of the changing perception of color throughout history reminds me of my favorite segment of Radiolab, which covers that very topic.