For a stark example, post #1304457 (rating:Q); as another, post #825641.
While this seems it'd generally coincide with the dark_skin tag, it could just be any time the nipple/areola hue(?) is lighter than the surrounding skin.
Since a more typical color for a light-skinned subject's nipple could be on a darker-skinned subject, I'm hesitant to go with pale_nipples, and light_nipples is antonymous to dark_skin and, appropriately, dark_nipples.
Updated by jxh2154