Hi Cecilia,
Great Peeps Quilt!
What an original and great way to come up with the pattern.
I would use one of your other colors for the narrow border instead of the white. (White is a receding color) The darker color will help to frame your blocks and set them apart from the outside border. Your eyes need a place to rest from the blocks before looking at the outside borders.
You could also use each of the colors to make multiple borders. If you like the yellow being the biggest that would be great. I would also then have the binding match the outside border if you have enough fabric. If not then I would make different lengths of all of the fabrics in your quilt and have a scrappy binding for your quilt.
I would preview the fabrics at the edge of your quilt in the approximate width you think you might want each border to be by just laying the fabric on the floor or design wall.
Pam M