Causes of Tooth Discoloration

The crown of a tooth is made up of two mineralized layers. Enamel is a white, heavily mineralized, porcelain-like, external layer. The inner layer, dentin, is the yellowish, less mineralized, supporting layer. Over time, the enamel breaks down and micro-fractures can form in the outer layer. These micro-fractures pick up stains, debris, pigmentation, and start to show the yellow dentin support layer. The degree of staining over a lifetime is influenced by foods (curry, coffee), drinks (red wine), habits (smoking, teeth grinding), and medications (Tetracycline). There is also a genetic component to the color of your dentin that affects how white your teeth are.

Credit: http://www.thirdstreetdental-drho.com/teeth-whitening-vs-bleaching/