After feedback, both here and on a thread on NGRM Online, I've had another go at painting rocks.
The two new rocks are at the front. Both were painted with a less diluted mix of stone gray and a thinner wash of black. The one on the left I used a much thinner black wash and focused it in on the details much more carefully. I think this works much better than the previous attempts as there is a nice base colour and plenty of variation. It might not be a perfect match but I think it's close enough to what I want and anyway lots of it will be covered in lichen, moss, and other gunk.