When a tooth that has undergone root canal treatment (endodontic therapy) turns grey or discolored, it is typically due to changes that occur within the tooth's structure as a result of the treatment. This discoloration is often associated with the loss of vitality in the tooth and changes in its internal tissues.

The grey line you're noticing around the edge of an older dental crown is likely due to the metal base of a porcelain-fused-to-metal (PFM) crown becoming exposed. PFM crowns consist of a metal substructure covered by a layer of porcelain. The grey line typically becomes visible over time for a few reasons, an older crown, a metal crown substructure, gum recession or decay.