The answer is this:

There is nothing in the source material that suggest race, or race relations in those worlds works anything like it does in ours. You're superimposing a non-existent social structure onto fantasy worlds. that's why black and brown people can show up when and wherever.

you think those white people in HotD or RoP are even white? They have no cultural connection to modern day real life white folk, so why would you think the racial dynamics or even the dna would work the same when the science (or magic) doesn't? It's non-nonsensical.
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"To Each His Reach"

but.....

Fuck aliens.