I am told foundry's new paints do very well for this sort of stuff, I think they released the three tones and looking at how the dad has used them on his stuff they are very, very good. So might be worth checking out. But yeah, I always have issues with skin tones, recently just painted skin a dark brown (often scorched brown from GW stuff) and then painted on some AV medium flesh tone, then went over it with a Citadel Devlan Mud wash and then highlighted some features with Dwarf flesh... But, you still have to hope the paint goes on okey.
And yes the Dark skinned trooper is definitely an accomplishment unto himself, tried darker skin tones earlier on some figs, and it didn't turn out right. So nice going on that one.