I think I saw a mod enabling witches to wear their normal clothes somewhere. Let me hunt down the link for that.
EDIT:
I located a mod by Carrigon that does both as you requested. This mod prevents Sims from switching to witch clothes and skintones when they're turned into witches. You will have to register there to pick it up:
No Witch Skin and Witch ClothesI also believe this prevents Sims from losing their custom skintones and/or custom body meshes such as Athletic, Rio, Warlokk for example.
There are also a couple of other mods, one done by HystericalParoxysm at MTS2 and one done at MATY. I have no doubt there are several more mods out there.
If your Sims already are witches and still wore witch clothes, you can always send them to a dresser and have them Plan Outfit - Everyday and then change into their everyday clothes.
Thread closed.