Originally Posted by The Question
I don't think thats how it works. I think it's divided along concept, not look. I doubt either company specifically laid exclusive claim to the character's ethnicity.
That might have just been an aesthetic choice because they thought the green hair looked silly in the context of the film.
Not ethnicity per se but design rights, and physical representations which might have been trademarked and included in those IP rights which Fox were assigned. The use of Ronan for Scarlet Witch (who looks decidedly different from the character) might be not because they wish to change her ethnicity but that they have to come up with a different visual representation to distinguish it from a design which Fox might hold.