One of the reasons for a tooth appearing pink is the presence of External Cervical Resorption (ECR) or Invasive Cervical Resorption. This type of resorption has been described and classified by Heithersay.
Some of the predisposing factors for ECR include: trauma, orthodontics, periodontal therapy, surgical procedures, intracoronal bleaching/restorations, etc. For the reasons that are poorly understood, odontoclastic activity below the epithelial attachment would cause resorption that advances inwardly and, if untreated, eventually result in significant loss of crown and root structure. Resorptive process does not usually involve the pulp tissue and in most cases the tooth stays vital. Teeth with Class 3 or 4 resorption have poor prognosis. So, the key to saving these teeth is to catch the resorptive defects early on.
One of the telling signs is the presence of pinkish discolouration in the cervical area of vital teeth. A resorptive defect harbouring a very touch-sensitive granulation tissue can be detected easily by running the sharp, curved end of the explorer below the level of CEJ. The treatment of the resorptive defect is open-flap restoration. The tissue residing inside the defect is usually very easy to remove as it appears detached. As mentioned before, the pulp tissue is unaffected by this resorptive process and therefore, root canal therapy is not needed.