I am an organized person with color. Scrappy, randomly placed just doesn't work for me. Search for Rail Fence quilts and you will see different kinds of layouts with color. The pictures help me a lot to define my color style.
https://www.bing.com/search?q=Rail+f...&ghacc=0&ghpl=
Since all of your sub blocks will be 6.5 inches square before sewing them into the larger 12.5 unfinished blocks, I would not sew them into 12.5 inch blocks. That will give you a lot more design freedom by placing them how you think you want them and moving them if they don't work for your design style. After you decide how you want the overall look, then make your 12.5 inch blocks. Hope I haven't confused you.