My problem with that is the automatic assumption that they SHOULD map to an existant human ethnicity.
(Even if I was to entirely agree that what we see in EqG ARE, strictly, humans.)
Your skin-colour range is a product - ultimately - of chance events across prehistory (just like all evolution) likely with some additional complexity factors due to sapience/sentience early society. Running the same experiment again is highly unlikely to give you the same results - unless Harbinger Probability Engineering is in question (and that opens a slew of other possibilities, among them that EqG's "ethnicity" is instead probability-engineered to match Equestria's - hell, that's a possiblity even without that; EqG is an alternate EQUESTRIA, not an alternate EARTH, let us not forget.)
I don't see the difference between and off-pink human, a brown human and a blue human. (Well, okay, the blue human may be slightly cooler.) So to me, saying Rainbow Dash should be... not blue (and this is why I'm having to be very careful here) is fundementally the same to me as, say, saying Jubilee should be caucasian-coloured. It's not quite up-in-arms outrage (that latter has been done - see the 1990s Generation X TV show pilot1 - not the worst thing they've done to her), not quite in spitting distance of the Nasty End Of That Line Of Thinking, but it is at least a raised "seriously" eyeglow roll. You may disagree, but from my outside, not-a-human, equalist perspective, that is exactly what it sounds like. Sorry if that disconcerts everypony (or worse, upsets anyone), but that IS how it feels to me.
1Actually, don't.