This is a common result after years of having the Amalgam (silver) fillings in our back chewing teeth. Over time, these fillings start to open up and leak- allowing the saliva and bacteria to seep into the space between the filling and the tooth. Another cause of this is the staining from the Amalgam fillings. The Amalgam slowly starts to stain the teeth and can also sometimes stain the surrounding tissue- causing what is called: "Amalgam tattoos". The stained teeth start to appear gray. This may be an indication that it is time to have those fillings changed and the teeth examined to confirm there is no decay underneath the fillings. I hightly recommend going in to see your dentist to have these teeth examined. You may want to consider having these teeth treated with composite resin fillings or porcelain restorations. Discuss all your options with your dentist. Good luck!
Dr. Parnaz Aurasteh