Place small quantities of icing in several small bowls. Add a little food coloring to each one and mix well. You will obtain the most vivid colors if you use paste food colors, which are available at craft stores and cake-decorating shops. If you’re concerned about synthetic food dyes, natural colorants can deliver an attractive look using concentrated vegetable pigments. To make frosting look festive, we recommend the natural food dyes from King Arthur Flour (kingarthurflour.com). For naturally colored sprinkles, we like the “Nature’s Colors” pastel shades from India Tree. Find this brand at cheftools.com.