Having yellowish teeth is not very uncommon and teeth stains accumulate naturally. When a child is born and grows, the real color of teeth is calcium-white irrespective of the race that it belongs  to. When our life wheel moves on and on, the actual white color disappears as the teeth stains are caused due to our poor habits such as chewing tobacco, smoking, not brushing the teeth after consuming food and so on. Whitening for yellow teeth is possible for those with harder stains too, but to maintain the white color, they have to give up the bad habits to prevent further accumulation of teeth stains.