What was client thinking?

Plaster it with a color in it. . Maybe a terra cotta. Plant fast growing plants like creeping fig. That will attach to wall. Just plant one each wall they grow fast. Have mounds of dirt brought in for corner to add palm and plants. A border with wood chips and other color plants like bird of paradise. Are nice, grow fast.

better than this https://www.houzz.com/discussions/our-new-retaining-wall-dsvw-vd~654309

It has been a couple of years, curious how you finished it.