ΔE 25.4, loose: different enough that you should pick on purpose. Atramentum Black is noticeably darker, much more muted than Mortarion Grime.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Atramentum Black is noticeably darker, much more muted than Mortarion Grime.
The same difference applies the other way round: Mortarion Grime is noticeably lighter, 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.
See Mortarion Grime on the colour wheel See Atramentum Black on the colour wheel
All matches: Mortarion Grime equivalents · Atramentum Black equivalents