Did you use the recipe where you add cream of tartar and cook it on the stove top? That's what I use and I haven't had any problems if I add the food colouring before mixing it all up and cooking. I've also added it after, when I was kneading the dough and it did come off on my hands, but once I worked it in it didn't come off when the kids played with it. If you do it that way, I'd start with a drop or two and mix it thoroughly before adding more.