The blue worked out well for the sashing! I barely had enough to put a border around the outside and had to piece all four sides. Luckily this is a fabric that literally doesn't show a seam if you are more than a few inches away!
As I wanted it to be at least 40 inches wide (it is 38 right now), I figured I needed at least one more border. I tried several pieces of fabric that didn't work.
I finally decided to go with a very thin strip of the yellow from the centers and then found this piece in my stash which seemed to be perfect!!