I usually pick a couple of fabrics that I think will work for borders. Then I lay them out and put the quilt on top so you can see what the borders will look like. After fiddling around with various widths of borders and colour combinations you can usually tell which ones you like best. I think blue and brown would work really well, with one narrow and one wider border. Maybe even one with the tan colour. Whatever you choose, it will be gorgeous. Your bargello is amazing.