My daughter is a Middle School Band Director, so I've collected lots of music-themed fabric over the years.
Fabric.com has a ton of music themed fabric. I would try to find a black/white fabric with small musical notes on it for the sashing. Something that's not super busy and gives your eyes a place to rest between the pinwheels.
There are several on this page of Fabric.com that could work for you.
https://www.fabric.com/SearchResults...6046a5e&page=4
I've also got a gold on gold music fabric that I used for a sashing that worked out well. Not sure where I bought it. It had small musical notes, but being two shades of gold, it read as more of a solid gold from a distance. That was for a t-shirt quilt I made for my daughter's former high school band director. I would agree with others to avoid colors that are too similar to any of your existing blocks. That's why I would suggest a black/white, or a gold/gold, or even a red/black simple print that will complement your existing blocks without being distracting. For an outer border I'd use a nice black print. Good luck!