When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.) We prefer gel food coloring for this recipe because just a couple of drops create vibrant results. Look for true shades of each color and avoid neon or pastels for the most pigmented design. For the fluffiest frosting, be sure there is no sign of yolk in the egg whites; separate each egg white into a cup before adding it to the bowl. Also start with a clean grease-free bowl, preferably glass or metal instead of plastic.