Homosapiens are uniquely adapted to to the Earth and the melanin in the skin is a natural sun block. Considering that the system the planet vulcan is in a trinary star system, If vulcans were anything like humans they would all be black. Since obviously the majority of vulcans we've seen are not they should have a completely different method of blocking the sun that doesn't involve skin color.
This is particularly peculiar odd since Star Trek considers evolution as factual.
Evolution isn't all that simple. There could be a multitude of reasons for the variation in skin tone, even in an Earth bound sense. Perhaps as simple as a recessive gene, a throw back from when Vulcans were darker skinned that only manifests itself along certain blood lines or perhaps every thousand Vulcan births or so, making the dark-skinned Vulcan a rarity, but conceivable.
And/or... Maybe there were two "homo-vulcanian" races co-existing on Vulcan millions of years ago similar to, but not necessarily the same as, Neanderthals and Cro-Magnons from our own prehistoric history. A second tribe of Vulcans whose origins are all together different, and who perhaps had deeper skin tones. Or even single species migrations throughout the planet from one point of origin, developing separately. In either case; as time marched on these tribes finally meet (again, or for the first time) and perhaps merged, mated, and evolved resulting in two (or more) formerly separate, genetically compatible, races now unified but still exhibiting some physical differences.
Or... Perhaps the "black Vulcans" suffer from a genetic skin disorder like a reverse Vitiligo, or a pigment condition similar to albinos on Earth.
Or... Considering also that apparently Vulcans have been a space faring race for the last few thousand some years (or many more), any number of influences could exist. Perhaps a Vulcan colony was founded a few thousand years ago on a sister planet in the same system as Vulcan and developed such protective skin alterations based on what ever environmental influences they may have encountered.
And... Spock mentions that perhaps Vulcans (and Romulans) are themselves ancestors of Sargon's race from "Return to Tomorrow" allowing for other unknown genetic influences to be at play. On that point, while possibly a bit more far fetched, any number of alien races may have had a hand in Vulcan development, in some way and not limited to genetically, beyond planetary conditions developing Vulcan evolution.
There's always realistic plausibities in a sci-fi universe, which may have been taken into consideration when they cast Tim Russ as Tuvok. He was simply perfect for the role first and foremost, after that everything else is left up to the imagination fueling playful speculation.

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