Technically my overlay willingly became an Illuminant, but yes. Powerful overlays are not that rare, uncommon yes, but not unheard of. I mean, people were recently greek dieties.

[Seriously.]

They may look human, but their ideologies hinge on the aspects of their particular color. They also have utter mastery of their color of magic, beyond that of any other mages skill with it.

Appearances change though, as they are simply inhabiting a mortal vessel for a time.