ΔE 18.8, loose: different enough that you should pick on purpose. Mortarion Grime is noticeably lighter, much more saturated than Grimdark Shadow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Mortarion Grime is noticeably lighter, much more saturated than Grimdark Shadow.
The same difference applies the other way round: Grimdark Shadow is noticeably darker, much more muted.
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 Grimdark Shadow on the colour wheel See Mortarion Grime on the colour wheel
All matches: Grimdark Shadow equivalents · Mortarion Grime equivalents