Yup. I've been told to use the Phenol reagent (the red stuff) but I
think food color is probably a better idea.

Food coloring is supposed to be non-toxic, but maybe they just haven't tried on enough people yet to be sure, or maybe they didn't give the test subjects enough. Everything is toxic if used to excess. But food coloring seems to me to be the safest, easiest, and cheapest way to go.