So I finally made the decision to go with solids because I wasn't able to find marbles or any other fabric in 12 progressive shades like I need. It was pretty hard to find the 12 solids, and they're not perfectly matched, but I think they will work. Here they are. I got 10 of them from Hancock's of Paducah (I bought many blues from them and returned many, as well--they were great to deal with)and found 1 in my small stash, and 1 in Walmart.