The colour indigo owes its position in the colour spectrum to Isaac Newton, who established Indigo as being the 7th colour. Indigo is neither a primary nor a secondary colour. The human eye is hardly capable to detect the nuances of Indigo, so it takes a specific sensitivity to mark the colour variety. However Newton added for reasons of perfection indigo to the set of 6 primary and secondary colours (red, orange, yellow, green, blue, violet) to have the spectrum in accordance to music (7 notes), the seven heavenly bodies (planets and sun and moon) who were then known, the seven days in a week.
So indigo is the light that was missing to make it perfect, and that is Indigo’s mission.