GW washes. Base coated white,t hen successive layers of wash. I normally paint my big units directly, but as I wasn't planning to use those rat ogres, they made a good test subject.
Weathering powders from Forge World,
http://www.forgeworld.co.uk/Modelling-Supplies
I used to use a stipling technique for rusting effects, but after seeing how easy and effective the powders are. I switched.
They are awesome. They come in the really old GW pots, mostly stuffed to the brim (I had one which wasn't, but still pretty full).
They last a long time. I used them on a unit of 20 night runners , and can hardly tell there's any missing.