do you have a habit of drinking tea of coffee? or do you suffer from some medical problem such as hypothyroidism? or does any one in your family has this problem. This questions are relevant because it has been observed that even after proper brushing if a person suffers from some medical problem or if the intake of tea or coffee is high then also the teeth with due course of time turn yellow. Tea or coffee contain caffeine which results in yellowish teeth, also termed as intrinsic stains in dental terms. Similarly, heredity can also be one of the reason. You can visit a dentist
and go for bleaching
of the tooth or veenering of the tooth.