This green won:
It really brings out the little bit of the pale green in the square and makes the potholder top look more like a flower, which I like. So I have decided on the backing. Now I just need to decide if there is any embroidery I want to do before I attach the back or if I want all the embroidery to be after I attach the back. Right now I am leaning strongly towards only doing embroidery after everything is all put together.