Mortarion Grime vs Black Wash

How close are they?

ΔE 22.3, loose: different enough that you should pick on purpose. Black Wash is a little lighter, much more muted than Mortarion Grime.

Can I use Black Wash instead of Mortarion Grime?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Black Wash is a little lighter, much more muted than Mortarion Grime.

Can I use Mortarion Grime instead of Black Wash?

The same difference applies the other way round: Mortarion Grime is a little darker, much more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.