I would sash in a light color, not black, with yellow cornerstones. I would also replace the two really dark blocks, and maybe the 3rd dark print as well. I can't see it with a white outer border, would rather have something like one of the grey and yellow prints. The border of a quilt gets the most handling, and white will show wear more, and might look rather plain compared to your beautiful pieced blocks. (Of course some fancy quilting in yellow and grey might fix that.) It's going to be a pretty quilt