The two colours used for the base are un-diluted Burnt Umber 11027 and Charcoal 11086, mixed in a ratio of one part Burnt Umber to five or six parts Charcoal. (The exact mix was varied to give a more natural stone colour). It needed up to three coats to get a good even coverage.
I have also washed the whole structure with a wash of five parts distilled water and five parts Klear, with a spot of flow improver and a dash of dark brown and black acrylic paint.
I realise that the whole project is well behind schedule, but I am pleased that it is still moving along and keeping me interested.