Newton invented "indigo" as a rainbow color because he thought it would be more "esoteric" to be seven colors instead of the six there really are. Indigo is either a very dark purple or blue, not a color on it's own.
There is a real mystery color though, and it is brown, which is not represented by the rainbow.
There are four ways to make brown:
The mixture of all three primary colors, red yellow and blue
The mixtures of secondary colors
There you go, you learned something new today