Between a Rock and a Hard Place

As well as water one of the main features of the diorama I'm planning is the rock outcrop that supports one end of the bridge. Now I have cast and painted rocks before but on that occasion I wasn't trying to match the rocks to a specific spot, so I decided some more experimenting was in order; yesterday I cast a bunch of rocks and today I've slowly painted up a few small pieces to get a better idea of what colours might work.

While there are five rocks I've used just four different, heavily diluted, Woodland Scenics liquid pigments to paint them; four used just a single colour for the base coat, while one used a mixture of two colours. Starting at the front we have a small rock painted with black. Behind it on the left the rock was painted with concrete, while the rock on the right was a mixture of stone gray and black. At the very back on the left the rock was painted using just stone gray, while on the right I used earth undercoat. Once the colours had dried they were sealed with a thin layer of scenic cement and then a wash of black to help highlight the details.

I think the rock at the front that used just black is probably the closest to what I need although it needs lots of lichen and gunk adding. I'm not convinced by the two tone rock (second row, right) and the concrete is way too light. The stone gray looks good but I think even with dirt added etc. would be too light. The rock I coloured using earth undercoat is obviously very wrong for what I'm currently modelling, but if I wanted a sandstone outcrop I think it would work quite well.