"Slate Grey" is simple to make yourself... but the truth of how well you like the color is up to you... here is my formula
50% Flat Black
10% Forest Green (or Dark Sand, if you want an "earthier" tone)
30% French Blue
10% White (this can be increased to lighten for dry brushing if desired).
Makes a very nice "slate grey" that is useful for stone floors, streets, and for making a thinned wash of grime...