I use light color backing and I quilt backing side up because I always do all over squiggles. As a main thread, the one that will show on the backing I use variegated thread, and In the bobbin I use whatever is neutral and will not be too obvious on the top. This is fairly unorthodox approach to quilting but I like it. As for the pieced backing I try to stay away from it if I can since I can never line the seam just right. If I absolutely have to I use wider seam allowance and press it open.