This is GORGEOUS! As for borders, I think they serve the same purpose as the mats around a photograph or other framed piece of art. (Lots of ways to google for examples.) In general, I like a very narrow first (inner) mat, followed by a wider second (outer) mat in a contrasting color. One might be the lightest color you've used and the other the darkest. The inner border should probably be either the lightest or the darkest, but the outer border could be whatever color you want to bring out in the quilt. Often a medium color is best - or a fabric that blends most of those used in the quilt. And don't forget that you get another chance to build the "picture frame" when you add a binding! (Maybe use the lightest color as the thin first border, a medium color blend fabric as the wide outer border, and the darkest color for the binding?)
Just my $.02