This isn't quite true. There are some foods that can and do stain excrement. Beets and red jello are famous causing people to think there is blood in their stool.
The reason it all seems to be roughly the same colour is that we eat roughly the same foods every day. By this I mean, the colour schemes of our food are basically the same colours we eat every day.
However; if, you were to go on a binge and ate only orange foods (for ex.) - carrots, pumpkins, etc. you would notice a change in the colour of your excrement.