Removing surface stains is the most common way to whiten teeth, but abrasive methods won’t do anything if you have a single tooth that’s darker than the rest.
A lone dark tooth usually results from trauma that cuts off blood supply to the tooth and leads to the death of the nerve. In these cases, you need to make sure the tooth has been treated from a root canal perspective.
A dentist can perform internal bleaching by placing a whitening agent inside the tooth. This will physically change the color of the effected tooth’s underlying structure.
Another option? An entirely new smile with veneers, or simply a single crown, which can hide the discoloration.