If you've been using the acronym ROY G. BIV to remember the colors of the rainbow for your entire life, you've been living a lie. Indigo and orange? They're both afterthoughts. Back when Isaac Newton first divided up and labeled the color spectrum using prisms, he settled on five main colors: red, yellow, green, blue, and violet. Later, he added in orange and indigo. Why? Because of music.
"Technically speaking, there aren’t seven distinct colors in the rainbow," design critic Jude Stewart recently told The Atlantic. "But Isaac Newton felt pressured to name seven colors to match the seven tones in Descartes’s musical scale–so he shoe-horned indigo in." Stewart wrote the book Roy G. Biv: An Exceedingly Surprising Book About Color to talk about the use of color and symbolism.
In addition to dismantling our common understanding of the rainbow, Stewart wrote about the use of color in different cultures. Most of the book follows the color of the rainbow, but she added in white, pink and brown, colors with plenty of their own stories. The book ties colors to some singificant cultural touchstones, asserting orange is violent (one piece of evidence: A Clockwork Orange) and brown is the color of death (evidence: feces, falling leaves).
She also answered a question some people may find important: How many shades of gray are there, in fact? The book has a chapter on gray titled a million shades of gray, but it doesn't provide an exact answer. But the number is, at the very least, 150--that's how many shades of gray paint Benjamin Moore sells.