There are several reasons why teeth may be yellow, such as:

Oral hygiene. Maintaining white and healthy teeth and avoiding frequent whitening processes largely depends on the standards of oral hygiene. Brushing after every meal is essential to remove food debris that damage enamel and cause staining.

Age. Stains accumulate on teeth over the years, turning them yellowish. They can even become brown. On the other hand, over the years a process of enamel loss occurs that accelerates the process.

Smoking. Nicotine stains teeth by leaving brown deposits that build up over time.

Nutrition. Coffee, red tea and red wine cause stains on the teeth, as well as citrus products (orange, lemon), cola and vinegar; they erode tooth enamel, making them porous and more susceptible to staining.