For people who have a hard time being random, try the "brown bag" approach sometime.
Do a simple scrappy pattern, maybe log cabin with 2" logs. Sort your fabrics out into your light logs and your dark logs. For simplicity, make 2 1/2" red center squares for each block.
Put lights in one bag and dark in the other. Now draw a strip out of the bag and that is what you use for the next log. The only time you can return the strip back to the bag is if two identical fabrics are next to each other.
Try making 6 blocks at a time. When the blocks are all done, put the 6 together in your predetermined layout. Then go back and make your next 6 blocks and add on to your quilt. Continue until the quilt top is completed.
Don't decide on borders until the center is completed.
You will be surprised how happy you will be with your quilt and the mix and match is all done for you!
Barb