Weren't there other Osh'Teks in story mode? Isn't the color of his paint/skin based off of
Maya Blue?
And the comics have had inconsistencies, embellishments and gap fillings in comparison to story mode so I wouldn't be all "/thread" bringing it up. However, the game developers probably agree it's paint since the Osh'Tekk warrior skin is a more normal tan pigment with white paint.
Personally? I would prefer it to be paint. An overgrown smurf isn't my cup of tea especially when he's based, in part, on the culture of my ancestors (Mayans).