Coloring rice is easy-peasy. Put your rice in a bowl, dump some hand sanitizer or rubbing alcohol (a little goes a long way) and put a couple drops of food coloring in. Mix until you have your desired color!
Let dry for an hour or so (just to be safe!) and make a cute sensory bin! No need to be fancy. Just add some cups, funnels, spoons, etc, and let your child explore the fun texture!