Why is p**p only brown?

Question

Why is it brown? Why can't it be purple?

Answer 1

Brown is a derivate of green, the colour of bile. p**p can be black, red, purple, depending on what you eat, Or almost white, if you have gall bladder poblems.  The only variation I have not seen is transparent.

Answer 2

It is not only brown.

The color of solid (fecal) waste is largely the contribution of color degradation products such as heme.  Heme is the non-protein part composed of a molecule called protoporphyrin whose structure binds to iron (Fe2+) which is the metal involved in oxygen transport.  The breakdown of heme includes products like bilirubin and biliverdin, which are excreted in the bile and eventually color the fecal waste.

Of course there are colonic bacteria that make products that color the fecal waste.  The brownish color is just a coinicidence however.  If you consumed a non-toxic, non-absorbable, non-digestible purple colored substance, I have no doubt that you might have purple-colored fecal matter.