It takes a few thousand years for a population's melanin level to adjust to a change (which is usually by migration, not by the climate changing on top of them). And it adjusts not to temperature but to sunlight, which doesn't change due to global warming. And why would you care if they darken up anyway?

I do agree, though, that in the ways that you'd think environmentalists would usually be interested in, global warming would actually be a good thing, and it would also generally help the human condition too, because the main limit on both humans and life in general on this planet right now is that it's too cold, and coldness makes life hard. If global warming is real, then someday people will be sitting around in lands that are nearly uninhabitable right now thanking us for the climate improvement we so wisely and generously gave them, and discussing the correction of the icy weather problem as one of the greatest achievements of civilization and a monuments to our own greatness. The rumor/modern-legend that anyone could ever have been silly/crazy enough to be scared of it, as if it were a bad thing to unfreeze and bring life to frozen wastelands, will be one of those tales people tell each other just for the sense of amazement or humorous irony at how bizarrely alien and backward people in the past could be, but many won't even believe it because it just won't be believable.