Gosh, those blocks are pretty! I could see it either way -- either no sashing, or most any color you like!
If you choose a bright color, you could even use one color for the sashing and a different (also bright) color for the border -- for example, red sashing and blueberry or turquoise border. Or bright yellow/gold sashing and red border.
If you want a quieter look, then I think a deep but not dull red would work nicely.