Teeth appear yellow when the enamel starts to erode. Tooth enamel is naturally white but can be easily eroded. The next layer of the tooth under the white enamel is dentin which is naturally yellow. When the enamel starts to erode the yellow dentin begins to show and thus makes your teeth yellow.
White enamel is eroded by everyday food and drink. The biggest culprits are tea, red wine, soft drinks like coke, sports drinks and brightly coloured sauces. You do not have to stop eating these foods altogether, but cutting down and having a good oral hygiene routine will definitely help. Many people choose to get their teeth whitened to restore the whiteness in their smile and feel more confident.