I asked my SO about this because he is a chemical engineering student who is almost finished with his degree. He said without knowing what the food coloring consists of, he can only speculate as to why, but given what he knows about, well, all sorts of stuff pertaining to this, he says that it's likely the food coloring doesn't freeze as quickly as the water, and so as the liquid freezes, it freezes from the outside in (as many of you noted), pushing the food coloring to the middle. So for those of you who thought this might be why, good work.
He was actually really excited about this question.
I'm pretty sure he still wants to bust out his engineering pad and do some calculations.