Depends on the setting I'm running, though broadly speaking I've always followed the descriptions in the PHBs where dwarves have darker skin, rather than the art, where they're typically viking-pale, and have always enjoyed elves having weird skin tones like pale blue.
Drow I tend to do the reverse and follow the art and describe them as "slate-grey-skinned" or the like, or greyish-purple.
I don't run Tieflings as all looking like Hellboy/the Lord of Darkness from Legend, but rather go with 2E-style variable appearances, so it varies wildly there.
I'm always open to suggestions from players, of course, and I don't think I've ever had to say no there. The most outre request was for some kind of lizardperson with rainbow-sheen scales, which seemed fine to me.
Human-wise the first settings I came across and was strongly influenced by (Taladas particularly) featured a lot of non-white characters, and I grew up in London, so it didn't even occur to me to have most NPCs be white - rather they've always been pretty varied, albeit not necessarily matching up to RL ethnicities at all.
(As an aside, on the "science means underground races should all be pale thing", I think there's bit more going on with D&D there, and legit to not follow that. A sightless race who have always lived in total darkness and don't have darkvision, nor interact with things that do, would indeed presumably be pale, but any other group of creatures, especially ones that only went down there in the last few thousands of years, and haven't necessarily been subjected to selective pressure so severe that skin pigmentation becomes an issue, might not be. I mean, for starters being able to live underground means you must not need to be pale for vitamin D's sake, because your vitamin D - if your species even needs it - must be coming from somewhere else.)