Melanin
As in it's literally what melanin does. If you're not "white" as in having the lowest levels of melanin in humans other than complete lack of (albino), it tends to cause your skin in certain places to be darker than the rest of the body. This includes but isn't limited to lips, genitalia, elbows, knees.
You also see it in Asians as well. Basically anyone with a significant amount of melanin.