Darkening of a 3-year-old child’s teeth after knocking his or her incisors is most often caused by pulpal necrosis of the teeth due to trauma. After a child’s tooth has been damaged, many parents will observe a darkening of the tooth, which may be black, yellow or brown. These conditions usually indicate a problem with the nerve, or pulp, of the child’s tooth. Once the nerve is necrotic, this color change occurs as the tissue breaks down and metabolites seep inside the tooth. This may result in localized swelling and pain in the gums, which may interfere with the eruption of future permanent teeth or even lead to malformations of the permanent teeth and the need for root canal treatment. Therefore, if your child experiences a darkening of the color of his or her teeth, he or she should go to the hospital in a timely manner in order to clarify the cause of the problem and treat it as soon as possible.