Sateenkaaren unohdettu väri

Isaac Newton, when describing the colour spectrum of a rainbow, listed 7 colours: Red, Orange, Yellow, Green, Blue, Indigo and Violet. But today Indigo often gets left off this list of rainbow colours; leaving the 3 primary and 3 secondary colours we are familiar with.
(https://www.winsornewton.com/na/discover/articles-and-inspiration/spotlight-on-indigo)