If you do decide to try and color the white bark, you will need to purchase candy coloring, or powdered food coloring. Regular food coloring is water based and will cause the 'chocolate' to crystalize. Rather than 'painting' with the chocolate, you might want to put the chocolate into pastry tubes or plastic bags with a corner removed and have them draw with the chocolate. This technique can be found in many cake decorating books as a way to make figures for cakes. One could also use royal icing made with powdered egg whites to make 3-D figures that represent a story