How did you do your greyish stone?
My recipe:
* Black primer
* Heavy white dry brush mostly from top to bottom
* Wash with a cold black. I use 4 parts acrylic medium + 1 part airbrush thinner + Daler Rowney FW Payne's Grey; but a Contrast paint should give a similar result.
* Light dry brush from top to bottom with Vallejo Ivory (any warm white works) to improve definition
* Mix of washes and glazes with some earth-tone at the bottom to simulate dirt. I used Vallejo Flat Earth.
* Mix of washes and glazes with some vegetal green where moss would develop. I used Vallejo Deep Green and Lime Green.