Most of the same skin tones of real life modern humans appear in major D&D settings.
D&D 5e Player's Handbook p.29 says:
Human skin shades range from nearly black to very pale ...
Pages 30-31 describes nine ethnic groups of the Forgotten Realms, whose skin tones are variously described as dusky brown, tawny, fair, amber, yellowish-bronze, and dark mahogany.
The Sword Coast Adventurer's Guide, p.110-112, adds descriptors of tan, dark, olive, pale, tanned, bronze, and golden. This seems to cover most of the possible values of real-world modern humans.
The peoples of the World of Greyhawk are described in the D&D 3e Living Greyhawk Gazetteer as variously having golden, bronze, tan to olive, rich red-brown or dark brown, and fair. The Suel in particular have a higher than average rate of albino people.
I faintly recall Eberron's creator suggesting that humans born in unusual circumstances, such as during a planar confluence, might be born with rare skin tones, such as green or blue. However, I don't believe this was a canon article, and the official Eberron books tend to gloss over details of human skin tone.