It has been documented that 92.2% of discolored teeth are nonvital (dead). Nonvital teeth often lead to abscesses and pain.
In the center of a tooth is a canal whereby the pulp (vein, artery, and nerves) is found. When the tooth sustains a concussive injury, the pulp swells. The pulp cannot expand because it is encased in the center of the tooth; surrounded by the hard tissue of the tooth.
The blood supply is compromised due to the swelling of the pulp. This is called irreversible pulpitis. Most of the time, if not all of the time, the tooth dies.
Initially, pulpitis is extremely painful (we know this by asking people with pulpitis) then after the pulp dies it is not painful. After time, it often becomes painful again due to abscessing.
As the pulp tissue degrades the cellular fragment infiltrate the hard tissue (dentin) of the tooth thereby causing discoloration. Many of these teeth can be saved by performing root canal therapy.