Didn't want to hijack your thread, so I posted an example from my own gallery within the spoiler tags.
Is that the kind of effect you're looking for?
The way I did that was easy.
It's Scorched Brown, washed Gryphonne Sepia.
While that is still wet, you then apply a wash of Devlan Mud.
Using a wet brush, you kind of 'combine' the two in such a way that the color is gradually tinted to look far dirtier.
Let it dry, then wash it with Badab Black heavily watered down to seal the deal.