Over the years the NATURAL colors, deeper within the teeth, get darker. Also stains not only accumulate ON the surface of the teeth, but soak deeply INTO the teeth to further darken teeth. This is why at age 40 you'll see that your teeth are darker than they were when you were 16, even if you do not consume things like coffee, tea or red wine. But you'll see they're even darker if you DO consume those staining foods and drinks.
Teeth whitening works by using peroxide gels that give off oxygen. This oxygen, if it enters the tooth structure properly, breaks long-chain stain molecules into smaller pieces. The significance is that long-chain molecules in teeth are DARK. Little short molecules are white.