if it were me, I would go bold
But it really depends on how you want it to look at the end.
If you want a bold summery look: Go for a bright orange
If you want an airy look: go for light blue
if you want an earthy look: go for a green (can take it bright with a lime or more toned down with a grass)
If you want the sashing to fade and not stand out at all: go for a light grey, almost silvery
It would really depend on where you want to go with it.