I saw this insight on Glow Baby:

Brooklyn's poop is brown.

Ever wonder why is poop brown? Bile (a liquid that helps digest food) is mostly the reason why poop is the color that it is. Breastfed babies get lots of different proteins every day (depending on what Mom eats) which also adds to differences in poop color. And while formula-fed babies will get the same food daily, their variations in gut bacteria can also affect their poop color. So it's very possible that poop on Monday can look really different from poop on Tuesday.