Is there a reason you want to make this decision before you have the blocks done? I would at least do some blocks first, then try out some different colors. Sometimes what looks best is a color you never would have thought of. The "look" that you want isn't always easy to see until after you have a couple of blocks together. The blocks may be busier than you thought, or too calm, so at that point the sashing could change the look to more what you want.