Well I've been able to work on this piece a little bit more and I have continued with the walls. Last time I had worked on the brick work, now I finished off the plaster covering the walls.
The first layer of paint to cover the black undercoat was Vallejo Dark Sand. I went with a light color to help simulate the color of unpainted plaster. In this way when I dry brushed the colors over it some of the "plaster" would show through as chipped paint and aging. At least that is what I was hoping it would accomplish.