Untreated tooth decay can lead to dark spots on your teeth, which are cavities. Once a dentist has filled those cavities (using a tooth-coloured composite dental resin), your teeth have been restored. So why, potentially years later, are you developing yellow spots on your teeth? Enamel and Fillings The outer layer of human teeth is made of dental enamel, which is highly mineralised and incredibly strong. However, when this enamel decays, your dentist will fill any cavities with composite resin, which is designed to match the colour of your dental enamel.