It is not always brown - try eating fruit and vegetables only, and no coffee for about 3 days and then see what interesting shade of yellow/green it can be!!
Ok, some good responses, but you are listing the exceptions. You have to accept that it is nearly always brown.
If it is due to Haem, is it that the share quantity of blood cells that get recycled will nearly always influence the colour to be brown ?
Basically when blood is of no use to the body anymore, it needs to be expelled so it is dead blood cells that make the colour (duller red = brown). You think of allthe blood in the body, and how much is expelled daily. Mixed with food that the body has broken down (again, another grim colour if you think about it, ever make mixed up mud pies when you were a kid?!?!) the "dead" blood basically acts as a strong dye and makes it brown. Answered.