Good morning lybrate-user.
Reason for dirty teeth can be many usually based on Extrinsic or Intrisic stains.
If reason is Extrinsic (Improper brushing, Coffee
stains, chewing gutkha and all) it can be treated with professional Cleaning (Scaling).
If reason is intrinsic than it depends upon the severity of the problem and it can be treated with bleaching
of the teeth) or masking the tooth/teeth with Veeners.