You gotta remember your color addition. The plants are already green, so if you add red to the reservoir you don't really get red leaves or whatever. You get red plus green which is really dark. You can change the lighter parts of a plant more.

I don't think the food coloring would be bad for you or anything, but I'm not sure how well it would work long-term. Plus, you might lose the color when you flush. Not sure there. There could be fading or changes in the color during drying and curing, too.

It'd be interesting to see what happens though, I haven't read anything about someone doing this before.