The indigo color is referred to scientifically as a spectral color. It exists but the human eye isn’t able to distinguish it between the hues of blue and purple that surround it. Indigo encompasses both blues and purples, so to speak. If you look at the Wikipedia page for indigo, you will see both colors being referred to as indigo.
So indigo is a color that can’t really be seen. I think that fits very well with Equius, he’s the Heir of Void, after all.