In order to achieve whiter teeth, a number of people think they are not brushing hard enough hence, they apply so much force while brushing their teeth and make use of very hard toothbrushes.
The enamel which is the outer layer of the tooth is usually light yellow to grayish white in colour while that of the dentine-the inner layer is yellow.
The normal colour of the teeth is off-white and not fully white.
When you brush vigorously with a hard toothbrush, you wear off the outer surface of the teeth which is light yellow to grayish white thereby showing the colour of the inner layer which is yellowish in colour.
In other words, instead of getting brighter teeth, your teeth will end up turning darker/yellowish.
Remember, brushing very hard will not get you whiter teeth rather, the harder you brush, the more yellowish your teeth will be.
Take caution: brush with a medium textured toothbrush.