It is completely normal for a 21-day-old baby to have different colored poop depending on whether they are breastfed or given formula milk. Green poop after breastfeeding is usually caused by the presence of bile in the stool, which is perfectly normal. On the other hand, yellow poop after formula feeding indicates that the baby's digestive system is effectively breaking down and absorbing the nutrients from the formula. As long as your baby is gaining weight, has no signs of discomfort, and their overall health is good, there is no need to worry about the color of their poop. However, if you notice any other concerning symptoms or if your baby seems unwell, it's always best to consult with a pediatrician for further evaluation and guidance.