You can have a bit of pattern for the walls like say how in old school D&D maps they used to use pen hatching from the walls extending into the rock for a little bit. But it would be a bit of a pain to have a complex pattern extending into the rock a far way as I need to make a mask to extract the caves shape. For caves its not too bad anyways since ill just get out a thick brush and draw around them approximately and then fade down the edges so that the caves fade into black. If you have a solid colour as the deep rock then its easy to select the caves out of it. That is all the issue is so not a huge problem.