Newton "picked" seven colours (most people only clearly distinguish 6) because of Greek philosophy, music (seven notes, then next octave) and alchemy. He'd originally listed six! There are very many more colours than 7 in a rainbow if you have a good list of colour names, or a pantone chart. I think the limit is about 10,000 hues for humans viewing spectrum of sunlight on very narrow slit and prism.
Asimov thought Newton's choice odd