Beat yolks in small dish; divide amount 4 custard cups. Stir in food coloring.
For blue and green, stir 1/4 teaspoon of coloring into each cup. For all other colors, stir 1/2 teaspoon coloring into each cup. To make black, combine 1 whole egg yolk, 1 1/2 teaspoon green, 1 1/2 teaspoon red, 5 drops blue.
Use small, clean paint brush to pain designs on cookies before baking.