In that case I would go with a light or medium purple. Dark purple may be too detracting and very often dark thread on a white has a tendency to take over. Especially in a solid as is the case in your quilt. I would audition a purple very close in value to the ring colors and maybe a shade lighter like a lilac. Your choice in batting will also impact how much the quilting will show. Something like Hobbs Polydown will give much more relief and shadow to the quilting (thus having it show more) as opposed to something very flat like Quilters Dream select or Hobbs 80/20.