I did some research and found a natural way remove food coloring stains from skin. This is helpful for anytime you or your kids are working with food coloring, like dying Easter eggs.
In a small bowl, mix 3 Tbsp. of salt with 5 Tbsp. of lemon juice or vinegar to make a paste. Gently scrub the stain with some of the mixture. Rinse with warm water, and repeat until the stain is gone.
As you can see, the stains on her hands almost disappeared after this treatment. The fingertips were still slightly green, but much better than before!