In a small mixing bowl, beat together yolks and a 1/2 teaspoon of water.  

Divide mixture between all bowls.  

With a different paint brush for each, mix a few drops of food colouring into each bowl. Be aware that the yellow of the egg yolks affect the mixed colour results.

(If mixture thickens while being used, stir in a bit of water, one drop at a time.)

Paint cookies and/or pie crusts before baking.