My front teeth were damaged in an accident 20 years ago. They have all porcelain crowns with multiple root canals including metal posts. The crowns were redone about 3 years ago in 2016 and have always looked good as my dentist did the proper build up to ensure they are not grey from the roots and posts. A few days ago, I noticed that one tooth has a large grey spot in the middle. This seems like it could be decay or an issue with the post. What could be causing this? How serious is it? Other than the grey spot visually being noticeable there isn't pain or anything else I can attribute to it. I am currently living abroad and was hoping to wait a few more months until I can get back to the USA for my dentist to look at it.