It is generally normal for a 72-day-old baby to have dark green stool. The color of a baby's stool can vary depending on their diet and the way their body processes food. Dark green stool in infants is often caused by the presence of bile, which is a substance produced by the liver that helps with digestion. As long as your baby is otherwise healthy and not experiencing any other symptoms such as diarrhea or abdominal pain, there is usually no cause for concern. However, if you are worried or notice any other changes in your baby's health, it is always best to consult with their pediatrician for further evaluation and guidance.