One of the things that Isaac Newton did whilst he was self-isolating from the plague in 1665 was to experiment with prisms. He shone a narrow beam of sunlight through one prism and got the rainbow spectrum. He was convinced that the colours were present in the light, but he was aware that the prism itself could have been behaving like stained glass. To rule this out, he took the spectrum from one prism and shone it into a second upside down prism. The colours joined together again to make white light. If the prism was like stained glass, that would have been impossible. So in self-isolation, Isaac Newton proved that white light really is made up of the colours of the spectrum. https://micro.magnet.fsu.edu/primer/java/scienceopticsu/newton/ As I understand it, he chose 7 colours because 7 is seen as a propitious number in religion. That's why I struggle to pick out blue from indigo from violet.
