Ay, normal lang yan usually, lalo na kung mixed feeding! A few of us mums noticed na ang dark green poop happens kapag may combination ng formula and breastmilk, or sometimes it's just how their digestion processes the nutrients, hindi alarming unless may other signs like fever, inconsolable crying, or blood sa diaper.
But honestly, para sa peace of mind mo, I'd still check with your pedia just to confirm everything's okay, especially since you're doing both formula and breastfeeding, they can check if the color matches your baby's age and feeding pattern!