Green poo - formula fed.

Hi so my second baby is Now upcoming to 6 weeks old and he has always had green poop whether it’s runny or like paste. Up to the first 2 weeks he was constipated for days off and on so I just took the green poo as build up’ but I’ve never seen his poo yellow the mustardy colour it’s supposed to be by now.

Doctors want to take a stool sample as they’re unsure what it could be, but at the moment they have listed it as a tummy bug/infection. And told me to keep him hydrated. But he does seems happy in himself. No high temperature. no sickness, no rashes etc.

I did read online that it could be the formula as he is on cow and gate first infant milk as it contains iron but I was told by 3 professional people that it’s unlikely to be that as most breast substitute formulas have iron in and if that was the case every baby would have green poo.

But I thought I’d ask if anyone can relate to this situation and if so what was the outcome?