Tooth turning grey for no possible reason with no pain or sensitivity. Any suggestions? (photos)

My guess is that the nerve of this tooth is dying. It probably has been changing color and your teeth whitening highlighted the discoloration. The lesion on the gum is probably a fistula. I would advise testing for vitality and possible Root Canal Therapy.

I would suspect that the nerve of the tooth died (necrotic).  These teeth can be asymptomatic for long periods of time and then one day they can develop a fistula (bump on the gums).  The dead nerve can cause the tooth to discolor, often grey.  If this is the case the tooth requires a root canal and most likely a crown.  Good luck!