A specialized non-toxic, water-based acrylic shade wash manufactured by Games Workshop.
Part of the yellow-green color family, it is primarily used by hobbyists to add natural shadow depth, weathered grime, and realistic dirt effects to battle-hardened tabletop miniatures, translucent, smooth matt finish when dry.
Low-viscosity "wash" properties engineered to flow off flat surfaces and settle cleanly into recesses to accentuate fine model details. It is recommended to saturate the model quickly using a large brush while the paint is entirely wet to avoid undesirable "coffee stain" rings as it cures. 
For pooling areas, use a dry brush to absorb the excess back out of recesses. It is advised not to mix this shade with Contrast Medium, as dilution breaks down the unique grime pigment concentration too drastically.