Canonically literally every Space Marine (except for the Blood Angels IIRC) should have dark skin when exposed to sunlight and then (except for the Salamanders) flash to an unnatural pallor when they step inside, because one of the mutations they get is photochromic skin to protect them from getting sunburns while still ensuring they get enough vitamin D. That sounds incredibly stupid and like I just made it up but that is the actual canon from the lore, along with Space Marines eating their enemies brains to gain their knowledge and having acid spit like the xenomorph.
Warhammer 40K is a very silly franchise and the art direction does it a disservice by not leaning into it more.
Also the Emperor is from somewhere in what is now Turkey and canonically the most common language the humans speak ("Gothic") is descended from Mandarin and is just artistically portrayed as being Latin based because "High Gothic" is supposed to scan as archaic in the way that inserting incorrect Latin into English does.
I think that even got removed from canon. It wasn't even necessary tbh the point of the grey knights is they are downright impossible to corrupt. Hell they have kaldor draigo roaming the warp for hundreds of years (although hes like a gary stu)
I keep saying 40k is over the top and even that was just goofy. The emperor gets fed 1000 psykers a day to keep him going. Then I remember covid coming along and over 1000 were dying a day and it was seen as acceptable sacrifice to grease the wheels of this machine and suddenly you realise FUCK
You have a source for Gothic being Mandarin-descended? Never heard that before, but I could see it. The imperial palace being in the himalayas would certainly put it geographically in the correct place, but I don't remember from where he started the unification wars.
I think it's something from an old Rogue Trader article? So it may be more a metacommentary from an old writer sort of thing, that Gothic would have been descended from Mandarin with a bunch of English vocabulary mixed in, albeit all unrecognizable after tens of thousands of years of sound changes.