Think of it in terms of what is making the wall dirty?
If it is fire then dark greys and blacks will be used. Washes of black and black pastel chalk as well as some airbrush work will make great soot.
Road dirt being kicked up by passing vehicles? What color is the road? Thats the color you'd use to "weather". Same thing with mud splashes etc. A toothbrush used to splatter along the lower areas of the walls will do the trick.
Grime and smoke damage? A dark wash, then some dry brushing. I always look for downspout stains, rain damage, rust from exposed pipe or fixtures, mildew etc. All things that will add charachter to the building. Vines, weeds, grasses (from where birds have carried them up to outcroppings etc. ) bird droppings etc. again are things to add for more realism if you choose to do so.
Mike
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"
