This is a lovely glaze with a subtle sheen
Recipe:
1 egg yolk
Few drops water
Paste food coloring
Use a new paintbrush to
add fine details.
1. Stir together egg yolk and water. Divide mixture among several bowls. Mix a little paste food coloring into each.
2. Use a small, clean watercolor paintbrush 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.
3. It’s 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.
How to Separate an Egg
Many of the techniques, such as egg yolk painting above, will require a separated egg.
To separate an egg white from the yolk:
Gently tap egg in center against a hard surface, such as the side of a bowl.
Holding a shell half in each hand, gently transfer yolk back and forth between the two halves. Allow the white to drip down between the two halves into a bowl.
When all the white has dripped into the bowl, place the yolk in another bowl. Store unused egg yolk, covered with water, in airtight container. Refrigerate up to 3 days.
Fancy Flours: Where Bakers Bloom 
There’s an easier way to separate an egg. Crack the whole egg into a small bowl. Gently lift out the yolk with your fingers. It won’t break – promise! Even small children can do this. It avoids the possibility of the shell piercing the yolk. I’m in Australia and our eggs aren’t any different from yours. Try it – it works!