I'm different than most as I prefer the lighter cornerstones. I feel like they connect the blocks and keep them from being totally overshadowed by the very dark sashings.
If you don't have the gold that is the center color of your sashings for cornerstones, perhaps picking up the color of some of the foliage... the golds I think would tie it all together.
My first vote would be for the center color of your sashings tho.