Your tooth enamel is naturally white and shiny, however as the enamel becomes thinner, to becomes more transparent and the inner layers of your teeth begin to show through. In this case it’s the dentin that is showing through and your dentin is naturally more yellow in colour that the enamel that covers it.
Sadly as we age the enamel layer of our teeth naturally becomes thinner. This is the main reason why as we get older our teeth begin to look increasingly yellow.