What could be causing my tooth to look grey?

Single tooth: Having one tooth turn grey, you may have a cavity in tooth which can darken. It will create a shadow. If the tooth has a silver filling, the tooth will stain darker from the metal. It can also stain the gums, a tattoo. Lastly, the tooth may darken from trama and necrosis of nerve. You will need a root canal.

Trauma/nerve death: Trauma or nerve death can make teeth turn dark.