If you have thin layer of enamel, the true color of your naturally yellowish dentin will show through. Age can darken the color of your teeth: As you get older, the outer layer of enamel thins over time, making teeth appear more yellow. ... The color of your teeth can also be affected by illness. How do I stop my teeth from being yellow? Follow good oral hygiene practices. Brush your teeth at least twice daily, floss at least once daily to remove plaque, and rinse with an antiseptic mouthwash at least once a day to kill bacteria that cause plaque. Use a whitening toothpaste (once or twice a week only) to remove surface stains and prevent yellowing.