Go to a dentist's office - a quick consultation should be free and your 'options' will be laid out along with several possibilities as to why the tooth changed and what the impact for the rest of your mouth is. See the problem here is that if it is a dead tooth and has been dead long enough to turn colors - then that means that the inside has rotted, liquefied and all you have is a 'thin' shell of the outer tooth preventing all of that gooey nasty infections and posionous stuff from exploding into your mouth, inside of the gum and going down your throat. Sounds nasty? It can be fatal if things go the worst possible way. Root canal, build up and 'capping' after draining out the puss and goo is the best way to go. That is if there is enough solid tooth there. If not then perhaps a bridge or an implant - a fake one to replace the dead one. That is assuming that the yellow means that tooth is dead. To give you an idea of what discoloration of a tooth or teeth means, please look at this page: http://www.wrongdiagnosis.com/sym/tooth_discoloration.htm When you see all the possibilites laid out, you might understand why it is very important to have a professional look at it.

that was really helpful. thanks !! =)