When Humbrol produced their range of weathering powders, one colour stood out amongst the selection - Chrome Oxide Green.
It's a pretty vibrant colour and for railway modellers, one that you don't see in the sets aimed at us train fans. This is because (some chuffer enthusiasts might want to look away at this point) there are other hobbies. Military modellers for example, often have different requirements to our diets of sludgy brown and black.
In this case, the green (think powdered frog) pigment is used to represent rising damp on buildings. I've had the chance to prove this on a recent model and it looks pretty good. Just brush the powder over the surface working up from the base. How high depends on the level of decrepitude your model represents. Mine has probably been standing for 50 years so the green has reached well up the brickwork (don't mention that this is probably the wrong bond...).
The results look better than my quick snap shows. I'll not be using huge quantities for the stuff, dark earth and smoke are my favourites, but it really does the job when required.