Hi, I had this happen to me.
My tooth was grey just arround the gum line. I had no pain with it whatsoever. No cold/heat sensitivity, it wasn't loose, didn't bother me one bit. But the fact that it was gray REALLY concerned me. I went to the dentist and it turned out the tooth had been traumatized (they think due to my having braces when I was younger) and the root and nerves in it died. They said the tooth in essence was "dead"...which is why I wasn't having any pain or sensitivity. I ended up having to get a root canal to clean out the dead nerve so it didn't cause infection or spread into my gum. It was cake.
I used Dr. Ariana in Broken Arrow for my root canal. ###-###-####. They were fantastic! I would bring her to the dentist asap and have them evaluate it and take x-rays. Hope it turns out okay.