If the meringue feels too flat or runny, you can add a spoon of powdered sugar and whisk again until it thickens.
Divide the meringue into several sections, and whisk in different shades of food coloring.
Get your cookies and use a pastry bag to outline your cookies with a single color.
Use another color of meringue to fill in the outline.
Allow the cookies to dry for several hours until the icing is firm. Keep them in the fridge.