Okay I gotta ask-- why is it yellow?

Sorry if this is a gross question. My dd2 has GI issues and I have learned a lot about poop, and I have always wanted to know why breastmilk poo is yellow? : I mean we are all moms here and you know our favorite subject is baby poop.

The reason this came up today is that my dd is on a special medical food/formula where all the proteins are broken down into amino acids. I called the rep to find out why her poo was the way it was and was it normal for that medical food. She said it was normal, that it is even sometimes will have little seeds in it like breastmilk poo, and that it's normal for the child to go a few days between poos b/c not much is left over. That brought breastmilk poo to mind. She said that the green color I was seeing was bile b/c bile normall breaks down proteins, and since there are none in this food then the bile comes out the other end intact. And then I started thinking about why sometimes with foremilk imbalance or OALD the poo is green and maybe it's bile, too. And then I started wondering why it was normally yellow? Yeah I'm thinking a little too deeply right now. So does anyone know?