When I have done blocks of the month for guild I selected a fabric that I loved that had many colors in it and committed that fabric to appear at least once in each block so there would be some consistency throughout all 12 blocks. The rest of the fabric I used were colors from that focus fabric. All the colors don't appear in each block and I have the same color but in different fabrics that appear in different blocks. I used a consistent background color throughout the blocks. This year for the BOM I'm using 1930's reproduction fabric that I've collected over the years. By having the focus fabric or a theme (like repo 1930's) I don't have to be concerned about having enough fabric and I can use a lot from my stash. Some of what I used in last year's BOM which was paper pieced stars I only had small scraps but enough to do that one block.