Concentrate of 'light, medium, dark'. That way it'll work whatever colours and fabrics people choose. And also add a little advice if it's suitable (or not) for directional fabrics and/or large prints.
There is a book about mystery quilts. Can't remember the exact title, but Amazon should bring up the details. I'm too tired to look for the details. We found out today that the government inspectors (Ofsted for the Brits here) will inspect my school on Wednesday and Thursday. It's headless chickens time and almost everybody is flapping. Should be a fun week.