Health Mystery. (Babies' Poo)

So it happens that I've been having this thought or if you like this question for quite some time.

It is a known fact medically that all the nutrients needed by a baby for survival at least for the first 6 months is present in the breastmilk. Here lies my question, since the breastmilk is white (or whatever color name you give it) and that's basically all that a newborn takes, How come the excreta (poo) of a baby is yellow, orange or slight brown? What's the science, chemistry or if you like mystery behind this phenomenon? How come an injested liquid (breastmilk) produces a solid or semi-solid product in poo?

Most definitely, there has to be a scientific explanation to all these.

Please professional comments are welcome.