Green poop...

Kind of a gross subject I know, but I need some input.

My wee man, now 16 weeks, has been on formula since he was 3 weeks old (due to inability of BF - which is a whole other post!). Since then he has had quite dark green, firm BMs. We initially started him on Gold formula but then changed to just normal (we tried Karicare and are currently on S26). He has 5x200ml bottles a day plus about 60ml of water every afternoon. I have mentioned several times to my Plunket nurse and doctor and have had him tested for any bugs (none). He only "goes" every third day but otherwise is happy and healthy with no problems sleeping or feeding and good weight gains. I'm thinking there might be a problem with his iron levels.

It doesn't seem normal to me and am wondering if anyone else has had, or is having, a similar problem. Your input would really help.