I've finally got round to finishing the wall. It's had a coat of white Gesso primer and is now getting some colour onto it. The first was a dirty wash to try and take the white from between the stones. Then I took a damp sponge and applied a mix of grey, brown and green. If you put too much on you can just wipe it off again with the sponge. Working in artificial light after sunset means it is really difficult to tell how it is coming out so it makes sense to keep the first coat light and do the next in daylight.
Here is how one end of the wall is looking.