Very beautiful colors! Very soothing! I have done several bargellos and this is what I have found works best for me. Add a small (about an inch or inch and a half) border to 'stop the motion' of the Bargello. I generally use a dark fabric, but not one that is in the quilt or that color will 'leak' into the border. After you have that visual separation, you can audition other fabrics. In some bargellos, I've used a fabric from the quilt, and since you have that breaking border, the color won't overwhelm the quilt. I've also used more pattern and designs in the border that complement the inner bargello pattern. But, even in a regular quilt, I often choose the border after it's done, so if you don't have fabric yet, take your quilt to the fabric store and see what you like.. It will be lovely !