I always tend to like Black for sashing. It might be interesting to try to see how the black pinwheels will react to the black sashing. This is a WILD suggestions, but if you have any black and white strip fabric, try that which may look like a keyboard stripe. I always try various ideas and take photos of them all until one of the layouts just looks perfect. I also have the guitar fabric and haven't found the right pattern for the fabric. Good Luck.