there are certain thing you have to remember actually they becoming yellow or  original they are yellow.i if they are becoming yellow check your habits eg.consumption of tea coffee etc.or nicotine product. also you have to brush twice daily in proper vertical brushing direction do not apply too much force. kindly consult dentist regurly for teeth cleaning every 6 months.

This is a very common complain of patients. As you have mentioned that the teeth are becoming yellowish there is a possibility that the teeth are getting stained. Unfortunately the toothpastes in market does not help in teeth whitening. Hence,  you should get your teeth cleaned regularly from a dentist. After cleaning if you are not satisfied with the appearance you can go for professional teeth whitening or bleaching which shows good results.

You need to know whether it is the stains that has made them yellow or is it your natural tooth colour I.e. the colour of your dentin, that has made them look like the way they do. Consult a dentist every six months so that you know your hygiene is being properly maintained or not. Stains and plaque can me eliminated by cleaning and polishing your teeth.