Poetry compares teeth to pearls. But teeth unlike pearls, are exposed to a variety of elements in day-to-day life. All these exposures are bound to take its toll on teeth color. They are also called the sentries of the digestive system.

CAUSES
The tooth discolouration can either be on the tooth surface (extrinsic) or be present within the substance of the tooth (intrinsic).  
1. Intrinsic causes

In a tooth with an intact blood and nerve supply, the causes are:

Hereditary disorders.
Tetracycline medication.
Excessive fluoride intake in drinking water.
High fevers associated with early childhood disease.
 
The loss of blood supply and nerve supply can by itself cause discoloration of the tooth.
 
2. Extrinsic causes
Stains from tobacco smoking.
Bacteria producing colored pigments.
A salivary protein layer covering the tooth surface.
The tinted remnants of a certain membrane seen on the teeth surface as the teeth, initially erupt out into the mouth.
Tooth decay or a broken down filling.

SYMPTOMS
Intrinsic causes are usually seen at the time of teeth eruption. 
- In hereditary conditions like erythroblastosisfetalis and 
  porphyria the teeth are colored in a brownish 
  blue and a brownish red respectively.