Fabulous challenge quilt and a good candidate to practice your hand quilting as you will quickly learn how to navigate the bulk of multiple seam allowances with this, especially on your one focus block with the green tree. As far as color, as Tessagin posted, it all depends on if you want the quilting stitches to stand out or not. It doesn't matter really, all boils down to what YOU want and like. But the "mood" of this particular quilt seems to call for a blender so I think I would go with a taupe/gray thread. On some blocks it will stand out and on others it will blend right in. As I was always one to not like tying off and doing a lot of starts and stops when I hand quilted I always opted for the same color thread throughout. One usually tends to get a rhythm or flow with hand quilting and having to stop and tie off to switch thread colors interrupts the flow and rhythm of the rocking motion. Just my .02