You'll have to go back and read the rules, but normally you can asked they use certain colors or do not use "red" as an example but you can not say what block or blocks to use. Be aware they do not have to follow your suggestions, you send out fabric at your own risk. Most people try to oblige your requests though. You can write "I hope I get alot of star blocks back from this swap". Last Boom I sent out fabric and asked that they not use red. I still got 1 back that did.