What color should healthy poop be?

This depends on your diet.

A regular diet should induce stool with varied shades of brown.

A diet consisting of an overshare of vegetables, especially green leafy vegetables would give you green shades ranging from Olive to Army.

A diet with alot of Iron, say one heavy on the mushrooms would give you really dark stools, ranging from a really dark brown to an almost black colour.

This is what I've experienced so far, mind you. If there are other varieties, I have yet to discover them. By the way, if your diet is balanced, it should float.