I think your sashings or "paths" between the diamonds need to be a solid or a blender fabric so the eyes have a place to rest. Also it will make the colorful diamonds pop. What color you choose will be up to you. Black or navy would be good. Or a light color like white, cream or yellow. I would probably choose black. It will be beautiful whatever you choose. It seems like a daunting project to me so kudos to you for starting it!