Please tell me what can cause a tooth to turn brown and grey, as opposed to yellow?

Pulpal death: If the change is not due to staining from something you ate, it is probably indicitave of a tooth whose pulp (nerve) has died and is in need of root canal therapy.

Pupal necrosis: If the nerve of a tooth is traumatically injured but remains vital, tooth may discolor and require esthetic restoration. If nerve dies tooth can discolor, requiring root canal treatment, internal bleaching, and/or restoration. See your dentist who may want you to see an endodontist.".