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All the Roy G. Biv mnemonics follow the tradition of including the colour indigo between blue and violet. Newton originally (1672) named only five primary colours: red, yellow, green, blue and violet. Only later did he introduce orange and indigo, giving seven colours by analogy to the number of notes in a musical scale.[25] Some sources now omit indigo, because it is a tertiary colour and partly due to the poor ability of humans to distinguish colours in the blue portion of the visual spectrum.[26]

In 1666, Isaac Newton used a prism to disperse white light and produce the now familiar spectrum of seven colors--red, orange, yellow, green, blue, indigo, and violet--that he himself named. (Feel free to call them Roy G. Biv.) Other people had played with prisms before. What Newton did next, however, had no precedent. He passed the emergent spectrum of colors back through another prism and recovered the pure white he had started with. This simple exercise demonstrated a remarkable property of light that has no counterpart on the artist's palette--where these same colors of paint, when mixed, leave you with a sludge-colored glob. Further, Newton tried to disperse each of the spectrum's seven colors, but he found them to be pure. The human eye has no capacity to do what prisms do. Another window to the universe lay undiscovered before us.