I was also thinking about something to accentuate the "circles," so I like Kitsie's idea. I'd use a thread the color of your green. It will blend better than you imagine with the tan. You can audition the thread by taking a foot or two off the spool and laying it on the quilt. It will look a bit darker when you quilt since it sinks down into the fabric.
Be sure and post your finished picture.
Darren