Extrinsic discoloration — This occurs when the outer layer of the tooth (the enamel) is stained by coffee, wine, cola or other drinks or foods. Smoking also causes extrinsic stains.

Intrinsic discoloration — This is when the inner structure of the tooth (the dentin) darkens or gets a yellow tint. Causes include excessive exposure to fluoride during early childhood, the maternal use of tetracycline antibiotics during the second half of pregnancy and the use of tetracycline antibiotics in children 8 years old or younger.

Age-related discoloration — This is a combination of extrinsic and intrinsic factors. In addition to stains caused by foods or smoking, the dentin naturally yellows over time. The enamel that covers the teeth gets thinner with age, which allows the dentin to show through. Chips or other injuries to a tooth can also cause discoloration, especially when the pulp has been damaged.
  
In rare cases, children with a condition called dentinogenesis imperfecta are born with gray, amber or purple discolorations.

What are the symptoms of tooth discoloration?  Symptoms include stains on the enamel or a yellow tint I the dentin.  A dentist can diagnose tooth discoloration by looking at the teeth.  Some tooth discoloration can be removed with professional cleaning, but many stains are permanent unless the teeth are treated (whitened) with a bleaching gel.

What are some factors that can lead to tooth discoloration?

Foods and drinks such as coffee, tea, colas, wines, and certain fruits and vegetables (for example, apples and potatoes) can stain your teeth as well as smoking or chewing tobacco. Poor dental hygiene such as inadequate brushing and flossing to remove plaque and stain-producing substances like coffee and tobacco can also cause tooth discoloration.

Several diseases that affect enamel (the hard surface of the teeth) and dentin (the underlying material under enamel) as well as treatments for certain conditions can also affect tooth color. For example, head and neck radiation and chemotherapy can cause teeth discoloration. In addition, certain infections in pregnant mothers can cause tooth discoloration in the infant by affecting enamel development.

As you age, the outer layer of enamel on your teeth gets worn away revealing the natural yellow color of dentin. Some people might also have naturally brighter or thicker enamel than others.

Prevention
 
By making a few simple lifestyle changes, you may be able to prevent teeth discoloration. For example, if you are a coffee drinker or smoker, consider cutting back or quitting all together. Also, improve your dental hygiene by brushing and flossing regularly and getting your teeth cleaned by a dental hygienist every 6 months.

Brushing your teeth after every meal will help to prevent some stains. Dentists recommend that you rinse your mouth with water after having wine, coffee or other drinks or foods that can stain your teeth. Regular cleanings by a dental hygienist also will help to prevent surface stains.

If your teeth appear to be an abnormal color without ready explanation, the abnormal tooth color lasts despite following good oral hygiene practices, and if other symptoms are also present, make an appointment to see your dentist.

Treatment

Discoloration often can be removed by applying a bleaching agent to the enamel of the teeth. With a technique called "power bleaching," the dentist applies a light-activated bleaching gel that causes the teeth to get significantly whiter in about 30 to 45 minutes. Several follow-up treatments may be needed.

It's also possible to remove discoloration with an at-home bleaching gel and a mouth guard given