What block/edging would compliment Harvest Blend retaining wall?

Realize any digging to install a tree ring will end up damaging roots. Same with under planting unless with something perennial/permanent to eliminate constant soil and root disruption. With that warning, I'd not try for a contrast, but rather compliment.....same color if not close to the same material as the wall to tie together, not contrast. Let your under planting be the focal point, not the tree ring.