Is it normal that since a baby switched to formula milk, his poops are dark green and he does every other day, sometimes every third?

It is very difficult to assess what is normal and what is not for a child to take formula milk - babies react in a very individual way to the administration of the mixture. It also depends on the way of switching from natural feeding to modified milk. Milks are very different - there are at least a few hundred of them on the market - you did not write which one specifically receives the baby. I have not personally encountered a classification of what color and frequency of stool are normal when giving a child a mixture - in my opinion, this is a variable issue. If you are concerned about the frequency of stooling by your child, this should prompt you to talk to your pediatrician, e.g. by phone: https://www.gdziepolek.pl/blog/jak-telefoniczne-otrzymac-recepte-i-zwolnienie-lekarskie I attach a study that may be helpful: https://www.gdziepolek.pl/kategorie/mleko-modyfikowane