How pretty!! It's fun to look at all the different patches in there. :D I think any of the colors will make a great border. If you're worried about goofing the quilting up (a constant fear with me!), do you want to have a long arm quilter do it? That's my favorite option because I hate the sandwiching part. Or else keep it simple. Maybe go on the diagonal?
Good luck!! Thanks for sharing your project!