Teeth, including a single dark tooth generally darken in result of the absorption of key pigmented minerals, medications ingested during tooth formation or trauma. Surface staining is typically yellowish or yellow- brownish, and staining deeper within tooth structure appears more of a grey shade.
Food & Medication Tooth Staining: If a single tooth is only slightly off color, it is likely the tooth has merely acquired stain. Tooth staining can occur due to the foods and drinks we consume and specific medications we take. This can happen for a number of reasons including misalignment, uneven and rough tooth surfaces. If a single tooth is unreachable for toothbrush bristles, build up and stain acquire quicker than the surrounding teeth, leaving the tooth to appear yellow or yellow-brownish.
Trauma Tooth Staining: If the single dark tooth is the result of trauma the staining is caused by saturation of iron from the bloodstream during periods of inflammation. Staining caused by trauma is very deep within the dentin and among the most difficult to remove.
If the tooth has experienced trauma at any point, it is possible that the nerve inside the single dark tooth may have died. A dead tooth often has a dark or grey appearance. Discoloration after trauma is similar to a bruise on the skin, resulting from internal bleeding of the pulp chamber (the central portion of the tooth containing blood vessels). It is recommended to schedule a dental appointment to seek treatment for the dead tooth because the space inside the tooth where the nerve used to be can become infected easily.
Typically when a single tooth appears dark yellow or yellow-brown it can be bleached by itself using a custom-fitted tooth whitening tray to isolate the one tooth. Once the single dark tooth matches the surrounding teeth you should apply the bleaching gel to all teeth until they appear to not getting lighter.
If a tooth appears grey, it is possible that the tooth needs to be bleached internally via injection by a dental professional.
Both solutions are relatively simple and produce satisfying results; however not all situations are alike and unfortunately some may need to consider veneers or crowns.