Discoloured baby teeth

Baby teeth, also called primary teeth, are typically whiter than adult permanent teeth because they are more calcified. Baby teeth can become discoloured for many reasons, including:

• Inadequate brushing. If baby teeth aren't brushed properly, bacteria (plaque) might form on the teeth — which can lead to tooth discoloration.
• Medication use. Infant medications containing iron, such as supplemental vitamins, might cause stains on baby teeth. Taking the antibiotic tetracycline during pregnancy or breast-feeding can cause a child to have discoloured baby teeth, too.
• Tooth injury. A single dark tooth could be the result of bleeding within the tooth due to dental trauma.
• Weak enamel. A genetic problem with enamel formation might lead to discoloured baby teeth.
• Excessive fluoride. Regularly mixing powdered or liquid concentrate infant formula with fluoridated water might increase your child's risk of developing faint white lines or streaks on the teeth (fluorosis) if these kinds of formula are your child's main source of food. 

To keep your child's mouth healthy, avoid filling your child's bottle with liquids such as sugar water, juice or soft drinks and don't put him or her to bed with a bottle. Also, don't share eating utensils with your child. This can spread cavity-causing bacteria. If your child uses a pacifier, never dip it in honey or sugar.

Discuss your concerns about your child's baby teeth with his or her doctor. He or she might refer you to a dentist. After addressing any underlying issues, the dentist might recommend bleaching the discoloured teeth later on or simply watching the teeth for signs of other problems.

Source: https://www.mayoclinic.org/healthy-lifestyle/infant-and-toddler-health/expert-answers/baby-teeth/faq-20057765