Can you show an ingame screenshot? Not that I do not beleive you, but....
Not to mention:
As you can see, concept arts are, just that, concepts. The skin color may be different in the game.
Actualy, I do not see how Impa has dark skin. As I said many times before, the appearances of NPCs in MM are completly out of lazieness, so I doubt there is a connection. But it is possible.