The natural colour of your teeth is generally a light greyish to a yellowish colour. The teeth will naturally darken with age and will also be stained by different tobacco products and different foods you eat and drink.
There are different methods used for teeth whitening such as using surface whiteners or bleaching. Surface whiteners are products like toothpaste or chewing gums that use special abrasives with the ability to remove surface stains. Most of the bleaching products are peroxide-based and are able to alter the colour of the teeth. Not all teeth will respond to teeth discoloration treatment so it's best to consult with your dentist to see what the best solution is for your teeth whitening.