They are not actually white for teeth may vary in color from dirty white, yellow and even gray. Whitening teeth can be done as an out-patient treatment with the dentist assessing the amount of whitening that is needed and prescribing a specific treatment period for the specific case. This might involve a whitening tray which is measured through molding to fit your teeth exactly. This tray is loaded with a whitening solution in the form of a paste that is applied and left to soak on the teeth for a prescribed period of time. This can also be done in-clinic by the dentist and takes as short a time as it would at home.