Split Squares Quilt

Started this "crazy split squares quilt" and now I am going bananas, I am having problem what color would be the backing, I placed the batting 2" bigger than the top quilt and the backing will be at least 5" than the top quilt....so the size would be 50X70" but I do not have a fabric for the backing....perhaps I will mix 2 color of fabrics for the backing...any suggestions?

This is how I started, dark to medium color 35 squares of 11X11", 35 squares of 7 1/2 X 7 1/2", 35 squares of 5 1/2X5 1/2", 35 squares of - 3 1/2X3 1/2".  I fused them by zigzag stitch on the machine........