Um, it all depends what kind of food coloring. I would think that it would kill them, because food coloring has special chemicals that can harm your fish's' digestive system. It would go through their gills to their organs, and slow them down. It would also melt out their eyes and stomach when they drink. Fish are very delicate animals with very delicate organs. Unless if you want to be known as a fish murderer, I wouldn't try it!