Changing of formula

Hey everyone

New dad here with concerns...my wife has gone through WiC and each appt they changed the brand of soy formula. First enfamil..then gerber goodstart now similac soy. I see each contains similar ingredients while variations in things like calcium and iron. The reason for concern is when my son was on enfimil..stool was turd like and a yellow light green color. On goodstart it would vary from turd to splatter but it was a heavy dark green almost looked Grey and now on similac soy it's a heavy green but always splatter no turd. Question is should my child be on one brand of soy formula or if WiC changes it per month it's fine?