I would use a brown with some depth, not movement, but a rich, warm brown. Hope that makes sense. It would bring the other colors together and focus the viewer's eyes to the center. Given the beautiful colors you've used, you don't need to add any movement in your borders. I did a Turning 20 with about 3 years of Moda Xmas fabric. It turned out beautifully and I used a rich, red on red print that read as a solid for the border and it really made it into a gorgeous quilt. Just my two cents worth...