Tooth color is the product of two layers: a translucent enamel outer layer and an opaque, naturally yellow dentin core. What patients read as 'my teeth are yellow' is often dentin showing through thinning enamel, not a surface stain that a whitening rinse can remove.
Extrinsic stain
Extrinsic stains sit on or just under the surface of enamel and come from chromogens in coffee, tea, red wine, dark berries, curry, and tobacco. These respond well to mechanical removal (professional cleaning, whitening toothpastes with mild abrasives) and to peroxide bleaching.
Intrinsic stain
Intrinsic discoloration sits within the tooth structure. Common causes include tetracycline exposure during tooth development, fluorosis, trauma that damages the pulp, and simple aging. Peroxide bleaching can improve intrinsic stains but the response is slower, less predictable, and sometimes limited.
Age-related color change
Enamel thins gradually across a lifetime while dentin continues to lay down secondary dentin, becoming darker. This is why the same person's teeth look progressively more yellow into their 60s even with excellent hygiene. Whitening can partially reverse the visible effect but does not restore enamel thickness.