I usually use real dark artist's water colors. Here is a link that describes the procedure:
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=11687
You can also use acrylic or oil-based paints. Here is a link that describes the acrylic method:
http://www.swannysmodels.com/Weathering.html
In either case the "Sludge" will run into the corners and nooks and crannies and make everything look greasy and dirty.