The important thing to understand is that brushing doesn’t necessarily make your natural tooth colour white. The yellowish teeth could be due to two reasons. One is surface staining from tea, coffee, smoking, certain foods and plaque or tartar buildup. Surface stains can usually be removed with professional cleaning and polishing. The other is your natural tooth colour—enamel is slightly transparent, so the naturally yellowish dentin underneath shows through. if the yellowish colour is your natural tooth shade, whitening is what can make the teeth look lighter.