Oh they give you guidance for choosing your fabrics. Border Creek (the company whose mystery quilt I am currently working on) even gives you a sample colour way with the option to get that specific kit. When I saw the colours I was like "Ick!" No judgement on my part if you like them. It's just not my thing. Here is what they had to offer:
I learned about mystery quilts when I first started working at Serge & Sew in June last year. We were putting together plans for the previous year's Mystery Quilt Club's showcase party. The walls were lined with so many quilts of all the same pattern in any colour way you could think of. I was mesmerized. And I was determined to be part of it the next time around.
One thing I have learned is that it is very important to pay attention to the specifics of the instructions. When you don't know what the end result will be, you can't assume you understand how many of what you'll need.