How about just some satiny ivory ribbon? Sew metal circles to each end, or attach curtain hanger clips.
Or find a fabric that complements the curtains and your to-be-green walls...maybe something in green, or a pale green with black print. Tiebacks are easy to make; here's just one site with instructions:http://www.hutchal.clara.net/curtains/tieback.htm
Men's ties can also make good tiebacks, though I'm not sure this fits your decor.