Hmmm...I don't think there is a difference, but I've actually never really thought about the fact that their skin tone is indeed different.
What I think is, that Glukkons, much like humans vary in skin tone for the same reasons we do: the abundance or lack of sunlight in some regions. That may perhaps affect their skin tone in some way.
Of course, I can't be sure. Although Max and the other moderators will probably be able to elaborate.
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