Have you got a single front tooth that is discoloured?
Perhapse you knocked it when you were younger and it started to change colour?
It is easy to traumatise a front tooth due to knocks and falls and this may eventualy lead to the nerve in the tooth dying. This over a number of years can cause a tooth to become yellow or grey in colour.
There are several options for such teeth including crowns or veneers. Here is a case I improved by simply doing some single tooth whitening.
If you have a tooth like this and you would like to find out some options for treatment contact me on firstname.lastname@example.org
Click the link below to see my article published in Dentistry Magazine for single tooth whitening.