Last year we converted a 30 foot cliff into a multilevel access to the lake behind our home. Part of the new design includes a firepit area. The floor and walls of the firepit are built with giant boulders. My original idea was to grow ground cover (creeping jenny) between the rocks to add some texture and softness, but there's no irrigation system down there and the ground cover died out from baking for hours of sun on rock. Next we tried filling in the areas between the boulders with river rock, but every time we have high winds and giant waves (about once a month) it all gets washed out.

Does anyone have any ideas besides pouring concrete between the boulders? And if not, our landscaper suggested coloring the concrete; ideas as to the color? Orange-y to match the veins in some of the boulders, gray to match the boulders themselves, or dark brown to mimic earth/mulch between the rocks? Any ideas very welcome, thanks!!

Photo1: firepit seen from above, with dying ground cover between the floor boulders.
Photo 2: rock stairs leading down to the firepit area
Photo 3: boulders like these line the walls of the firepit and the surrounding areas