Food coloring can color things, although I'm not sure the extent to which they can color things not in an aqueous solution, and contact lenses are made out of plastic or glass, not water. Chemicals can change their physical properties (including color) when they change phase, so crystallizing a food coloring or dissolving it in another substance might not cause them to function properly. There is a long-standing argument about whether food colorings are toxic or not; some of them probably are and some of them probably aren't.

I found one example on the Internet of people dying contacts blue with food coloring, so it's apparently possible.But I asked my optometrist, and he said it's risky to put those contacts in your eyes. Some chemicals that are safe for your stomach may react with your eye, especially over a long period of time. Besides, food coloring is meant to dye foods, including meat--and we're made of meat. Don't risk it!