Hi mama! Dark green poop sa formula-fed babies ay usually okay lang, especially if consistent na siya and walang fever or other signs of discomfort, one of my mum friends' baby had the same thing at 2 months and doc said it was normal. But since it's been 5 days and you're noticing the color changes, mas safe na i-mention mo ito sa pediatrician during your next checkup or mag-call lang kung worried ka, kasi they can check kung may indigestion or kung ang formula ang agree sa kanya or not. Don't worry, first-time mom struggles talaga pero you're doing great observing na!