I would be tempted to not use sashing at all. But, if you want to, consider white sashing with assorted color cornerstones. The small colored squares might look nice with the diagonal lines of the blocks. I would probably do a solid white border the same size as the sashing and then an outer border of whatever color you want to feature. Or, perhaps an outer border of all the colors pieced together (not a piano border, a lengthwise one). Please post a pic of what you decide to go with. Really like the top, colorful and vibrant but not too busy.