I like the yellow (inner) and blue (outer) border fabrics. For the back, I'd piece leftovers from the top and supplement, if necessary, with similar or matching fabrics from my stash.
Also was wondering what it would look like if you substituted white for the yellow as an inner border, still keeping the blue for the outer? I've found that sometimes yellow can be overpowering if used too much; that is to say, a little bit goes a long way. Just curious...