Hey , Teeth generally are originally in different shades for different people . If your teeth naturally are yellowish by nature, brushing will not change the colour on the contrary over brushing can make them more yellow by removing your enamel layer thus making your teeth sensitive too . 1 ) If your teeth have deposits and external stains over them which makes them appear yellowish ... it can be removed by oral prophylaxis . ( scaling ) 2) if your teeth have internal stains like fluorosis etc then they can be treated by enamel micro abrasion or bleaching . 3) If your teeth are clean and naturally of a yellowish colour then we can do bleaching provided your teeth are healthy .