To be fair, I can't recall that Luke ever wore the standard brown robe shtick that all the other jedi seem to have going on. The only time I can recall him wearing any cloak at all when he first appears in Return of the Jedi, that cloak is black, and he loses it during the Rancor fight. I could be wrong (been a while since I've watched the movies), and I suppose he might have shown up in the brown robe and tunic in a comic somewhere, but it seems to me that if he wanted to be recognized as Luke his best bet would have been something that Luke actually wore, like the distinctively white farm clothes he starts with, a pilot's outfit, the tight blacks from RotJ, maybe his exercise clothes from Dagobah, something like that. Even given the hair, with that outfit I'd probably assume a young Mace Windu, too.
Again, I could be wrong, but it seems the confusion is likely more than just the skin color.