There are two types of discolouration of teeth.
Extrinsic - staining on the outer surface of the tooth. This is the sort of colouring that dentists can polish away. Extrinsic staining of teeth is caused by all sorts of things - smoking, tea, coffee, curry, orange juice and so on (even Corsodyl mouthwash stains your teeth!) .... But, hang on a minute. I personally live on the sort of stuff that stains teeth - I drink gallons of tea and I could eat curry every day for the rest of my life and be happy - and my teeth don't stain. So why not? Because my toothbrushing and flossing technique is good. If your technique isn't good, then your teeth will stain where the brush doesn't clean efficiently.
So the stain can be polished away. Preferentially, it would be done Professionally by a dental hygienist or dentist, and then your technique modified to try stopping it reforming. Staining of the teeth means your technique isn't up to scratch - so you're probably missing important parts of your mouth that may then suffer with gum disease or tooth decay.
So called Whitening toothpastes are basically more abrasive than an average toothpaste, Remember your Avocado coloured bath in the 1970s? The colour seemed to gradually fade, but when your dad took it out and smashed it up (to replace it with a trendy white bath) it was obvious that the colour was all the way through the bath. The bath cleaners of the 1970s were often very abrasive and scratched fine lines all over the surface. These lines scatter the light and give the impression that the colour has faded. If we take that analogy to toothpastes, the most abrasive pastes scour the surface to make fine lines. So they remove stain BUT also rub away the surface of the tooth, particularly if the surface has been softened first by acid, usually from the food we eat and drink - click this link to our page about acids in the diet. Ironically, if we scour the surface of the tooth enough, it starts to disappear (forever) and the darker inside of the tooth becomes dominant, so the tooth gets darker!
Intrinsic - discolouration of the tooth substance. A very different kettle of fish.
Teeth vary in colour. They just do. Some are light coloured, some are dark. If you're of African Caribbean origin, and therefore your skin colour is dark, your teeth will look lighter because they're against a darker background. If you're a Northern Irish redhead with skin so transparent that it's nearly blue, then your teeth will look darker. The same principle applies to lipstick. Wearing a darker shade will make your teeth look lighter.
Enamel on the outside of the tooth is the lighter layer. Dentine, deep inside the tooth is more yellow. Teeth with a larger bulk of dentine such as your canine teeth will look darker than the others - and that's normal. If you strip away the enamel with an acid diet (particularly with abrasion added) the tooth will get darker, instead of lighter. As teeth age, they become more translucent, so the darker dentine "shines" through more easily. Therefore teeth will naturally darken with age. If we then make them too white, you look a fool. The aim should be to give a nice set of teeth for someone of your age group.
Using peroxide based gels in a thin gumshield will tend to make the teeth brighter but also more opaque, turning back the clock without physically damaging the tooth. With experience the results can be pretty well predictable.
Some unfortunate individuals are given tetracycline group antibiotics when the teeth are forming. Tetracyclines are incorporated into bone and teeth as they form, this gives a grey or yellow discolouration of the tooth. If we go back to the predictability of whitening, the predictable thing about whitening teeth discoloured by tetracyclines is that they are unpredictable! The discolouration is at varying depths within the tooth and is of variable density. This makes predicting the result far more difficult.
Updated 16 Nov 2015
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