I know some people do make binding so that one fabric shows on the front and one on the back, although I personally haven't done it. I think the light would make a great final boarder for the front - or the darkest would make a great narrow boarder on the back. Rather than go through the making of a 2-color binding - which would have a seam right along the edge of the quilt, making it weak, I'd go with either of those.
Your quilt is gorgeous, I hope we'll have the chance to see it once it is quilted and bound!
There are a number of tutorials on how to accomplish a 2-color binding, if that's the way you decide to go. Here's one that looks pretty straight forward:
http://www.instructables.com/id/Two-Tone-Quilt-Binding/