Make the egg white cookies ahead so they have time to set. Decorate the the next day for a fun Christmas dessert.
Make cookies at least a day before serving, since they must set overnight. Heat oven to 200°F. Line 2 large baking sheets with foil; set aside. In large metal bowl, whisk together egg whites and sugar. Place bowl over pan of gently simmering water, and whisk constantly until sugar dissolves and mixture is warm, about 5 minutes.
Remove bowl from pan, and with mixer on high, beat in icing color. Continue beating until very stiff peaks form, about 8 minutes. Spoon mixture into large, resealable bag, and snip 1/2 inch off corner of bag. Pipe 1-1/2 inch mounds that bend to one side like Santa caps about 1 inch apart on prepared baking sheets. Bake 1 hour, switching sheets halfway through. Turn off oven, and let caps stand overnight.