Very few people have really white teeth. If your teeth are yellowish, it does not necessarily mean that your teeth are weak or unhealthy.
The bulk of teeth is formed by dentine which is yellowish in colour. This colour can be seen through the relatively translucent enamel.
Teeth darken with a person's age. This is thought to be due to changes in the structure of the enamel and an increased thickness of the underlying dentine.
My tooth looks grayish & discoloured. What can be done?
If the discolouration is due to defective enamel or dentine it could be masked by a layer of tooth-coloured filling or a veneer or crown of porcelain.
If the discolouration is due to decay, an old discoloured filling, or the tooth being dead, then the underlying cause needs to be treated first. For example, in the case of a dead tooth, root canal treatment may need to be done first.