[mannykannot]

Of the seven traditional named spectral colors, indigo and violet seem to be the least distinct pair. Newton may have included it mainly to have seven named colors, as a parallel to the seven notes of the western major scale in music. He admitted that his ability to distinguish colors was not acute, and, after he had selected his seven, he asked others to mark them off on a spectrum projected on a wall. This is different, of course, then asking them how many distinct colors they thought best characterized the visible spectrum.

https://en.wikipedia.org/wiki/Indigo#Classification_as_a_spe...

[OscarCunningham]

What I've heard as the explanation is that his "blue" referred to our "cyan" and his "indigo" referred to our "blue".

[aasasd]

Which almost exactly corresponds to the ‘rainbow colors’ as used in Russian (and probably other Cyrillic languages). The color swatches in the linked article are precisely Russian ‘blue’ for Newton's indigo and ‘lightblue’ for blue.