oh i love your (quilt!) Florence-
do you feel you need sashing? if it's necessary i would probably go with the navy- just because the (right purple) can be so difficult to get to play well with everyone else.
maybe even a white on white?...that would float everything. depends on your reason for sashing and how wide you plan to make it- navy would give distinct separation- white would float the stars with out as much distinct separation- i would lay out a few candidates & blocks, step back & see what i like best...looking good !
i need to get off the computer- go make breakfast- & get into my sewing room to make today's block!