Why, if white is the coolest “color” (it reflects all the light) and black is the hottest one (it absorbs all the light)—just compare walking in the beach with a white vs. a black t-shirt—, are people in sunny regions darker than those in less sunnier ones?

In other words, why isn’t being white (i.e., more light-reflecting) in sunny regions an evolutionary advantage? Whatever melanin^WP does (I think it’s supposed to be a sun-blocker), shouldn’t it do it better with the advantage of a more light-reflecting skin?