There is a whole spectrum of color, but there are only a handful of colors that most humans distinguish and talk about. It has always seemed odd to me that we elevate Indigo into the spectrum of named colors.
However, Newton was fond of the occult, and numerology, and the number seven. He wrote papers about appearances of the number seven in the Bible.
I think that Newton made Indigo an official color just to bring the number up to 7.
The seven colors are a consequence of us having 3 distinct photosensitive pigments in our eyes which aren't evenly spaced in terms of the wavelengths to which they are sensitive. If they were perfectly spaced we'd have 6 colors: Red, Anti-dark-blue=Yellow), Green, Anti-red=Cyan, Dark-blue, and Anti-green=Magenta. However because of the shift, what we consider to be true blue is somewhere between dark-blue and cyan, so we have Indigo which is halfway between True-blue and Red, Orange which is Anti-true-blue, magenta is blue-shifted into violet, and we forego cyan as one of the major colors.
Note that different cultures do not actually agree on 7 colors in the spectrum or which colors they are. For example some languages consider green to be a shade of blue or vice versa, or green to be a shade of yellow. Some only distinguish between hot (red-yellow) and cold (green-blue) colors. There are also some non-spectrum basic colors, for example english considers light-red (pink) and dark-orange (brown) to be basic colors, whereas a dark green is just a shade of green; and this also varies by culture.
There is a whole spectrum of color, but there are only a handful of colors that most humans distinguish and talk about. It has always seemed odd to me that we elevate Indigo into the spectrum of named colors.
However, Newton was fond of the occult, and numerology, and the number seven. He wrote papers about appearances of the number seven in the Bible.
I think that Newton made Indigo an official color just to bring the number up to 7.