I have painted in all three Barrel Cacti, added another Agave leaf to continue the crowed look, glazed a bit of Hansa Yellow over the agaves, and painted in the rocks.  Can you tell the hues used to create the rocks are combinations of  just the others used to paint the plants?  No black, no grey, just yellows, red, greens and titanium white.  There is some background adjusting to do as well as add the spines on the barrels, the stamen and pistils in the flowers.  Then the high and low lights will get adjusted.  But here it is as it looks today.