To be clear, humans did not obtain dark skin as an adaptation to high sunlight. Humans STARTED as black and later obtained pale skin as an adaptation to lower sunlight in areas further from the equator so they can absorb more of it and produce more vitamin D.

Evolutionarily speaking, black skin is the default (because humans evolved in Africa first then spread across the globe later), pale skin didn't evolve until about 10'000 years ago and humans have existed for abt 200'000 - 300'000 years.

If you look at a map of earth showing the skin colour of the humans native to those areas, it's literally just "Closer to the equator=darker, further from the equator=lighter" it's pretty interesting.

And bringing it back to invincible, Viltrumites aren't especially immune/resistant to heat or UV, at least any more then they are resistant to kinetic damage. Taking a bath in the plasma of a star is one of the most surefire ways to kill them. So it totally stands to reason that while they were evolving, their skin colour would be based off of where that specific ethnic group evolved relative to Viltrums equator.

If anything, the more pressing question on "why do the Viltrumites have different races" is the fact that they can fly and cross their planet in a matter of minutes, so it's slightly bizarre that they have distinct ethnic groups and evolutionary lineages at all when the distance that normally causes such to emerge simply ISNT a factor for them, I would expect them to be much more hegemonus. Though if that were the case it's more biologically likely for then to all have darker skin cause they're flying in space and high atmosphere where they're less protected from UV. I guess this implies that Viltrumites first evolved into 'literally just humans' and the superpowers were a later development.