So, it sounds pretty fitting for a bunch of lads in power armour who used to knock about calling themselves 'Legions' in the good old days of the Horus Heresy.
It's hilarious to me that you're going to claim that Romans were all white because 'muh Italy'. Fuck,
Italians were barely considered white until very recently, historically speaking.
Rome was an empire. Empires spread. And as they spread, the likelihood of them encountering niggers increases.
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Look at that shit. Why do you suppose we have niggers everywhere now?
Not Septimius Severus?

It's a miniatures game, dude. You can paint your Guilliman however you like.
I don't, actually. Similarly, I don't think a planet full of white people is particularly realistic. Why would planets in science-fiction be restricted to one race per planet?
Our planet isn't like that. There's bound to be racial differences, contingent on climate and location and all sorts of other factors.
For that matter, why would a different planet conform to the stereotypes of
our planet?