You really really didn't think this through. While they live deep underground and don't need to protect from UV light, they also don't need to have white skin t

You really really didn't think this through. While they live deep underground and don't need to protect from UV light, they also don't need to have white skin to let in sunlight for vitamin synthesis. In the real world, skin didn't get paler simply because there wasn't enough UV. It got paler because less sunlight meant that darker skin couldn't absorb enough light for synthesis. If this wasn't case, our skins would still be dark. So there is absolutely 1000% no reason at all why their skin would be any specific color. Dark or white, all are justifiable.