I usually make up silly rules as I go. For a scrappy string quilt made of 12 inch blocks. I put all my strings and pulled them out blindly. My rule for that one was I could not use the same fabric twice in a block. It turned out fun. Sometimes I divide my scraps roughly into "dark" and "light" and alternate them. This is especially fun for a string block made on the diagonal where one half is light and the other half is dark. I also used this light/dark idea for a scrappy jacobs ladder made with nine patches and half square triangle squares. in this one, I used any scrap as long as it was light or dark. In another one of large print fabric made into fairly large tumblers I just alternated "warm with cool" colors. That was fun. And then there was "scrappy" bed runner made up of "orphan blocks" from a lot of projects made over that years. That did require a bit more planning as the blocks were different sizes but I made them all fit by using "scrappy parts of leftover cuts from making blocks" Go for it. Just have fun. Make up whatever rules you want. I am currently working on a bunch of left over grey batick 1.5 strips in different hues and value. I decided to dig out my green scraps and cut some more 1.5 strips and alternate with the greys. This is actually being used in Bonnie's bitcoin pattern. It will just end up being a table topper.