Wash color

i went through the same question last week and got a few good peices of advise.....light greys or if your using oils a yellow ocher works as well,,,you can also do a pastel dusting with tans,or light grey,depending on which tone you prefer

I generally use a light gray, mostly neautral gray for sludge washes. Black is a bit stark, but gray is just enough to show shadow and a bit of grime.