When you are painting a rock formation there is a lot of work to do, a varied palette, the crags in the rock, the different tones of the different colours, the perspective, I could go on but I think that confirms it is a big piece of work.
To simplify this process I have a method I use which you may also find extremely useful. I collect all my old palettes, dating back to twenty years ago, watercolour palettes that is. Watercolour has such a wonderful quality of longevity. I keep all the palettes in my studio in a cupboard and use them off and on. If I need secondary or tertiary colours for rocks that is my source of colours. Never throw used watercolour residue away, unless of course it is a weak wash. I mix the residues which will naturally create purples, or browns, or dark greens, or a chromatic black.
So the palette is ready now all we need to do is paint. Have two pieces of cling film, ready to use. Apply three layers of the colours to the rock, e.g. brown, chromatic black and then purple. Let it soak in for around 50 seconds, and then apply a piece of cling film and then scrunch it so there are numerous creases showing. This is a bit like frottage in reverse. I then take an object that is heavy enough to press the cling film to the paper and I whack it on to the painted area. Leave it for around one minute. Then peel off the cling film, slowly. Apply your second piece of cling film and scrunch again. Peel off. You have a ready made rock with plenty of variation of tone and colour. I have used many objects over the years to create different effects, ordinary household items, something with a texture will give further interest.
In the painting above, for the cling film process, I used my mobile phone in its patterned cover as a press, the cling film box, a walking boot and I also whacked the painting against a wall, so each rock has a slightly different finish.