Stir together egg yolk and water. Divide mixture among several bowls. Mix a little paste food coloring into each.
Use a small clean watercolor paint brush to paint various colors onto unbaked cutout cookies. Clean the brush between colors using plain water. Put only a small amount of paint on the brush. If the Egg Paint thickens while you're working with it, stir in a little water, a drop at a time. If using more than one color, leave a narrow strip of cookie between painted areas so the colors don't run together. If you want a marbled effect, paint a small amount of one color on top of another.
Its important to use egg yolk paint only before the cookie dough is baked so that the raw egg is cooked during the baking step. Bake as cookie recipe directs.