When a primary tooth discolours and turns grayish, it is usually secondary to a traumatic episode. The discolouration indicates a necrotic pulp or that hemorrhaging has occurred and entered the dentinal tubules and can appear within a month of the original injury. The tooth may exhibit a light gray colour initially but may progressively darken. Occasionally, the tooth may present with a yellowish colour due to calcific degeneration of the pulpal tissues.
Treatment of these teeth is not always indicated unless there is evidence of pathology. As such, appropriate examination and follow-up is necessary in these cases.
Source: Dental Secrets, Elsevier, 2015