ΔE 12.8, fair: the same colour family, but not a straight swap. Mortarion Grime is a little lighter, more muted, warmer than Military Shade.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Mortarion Grime is a little lighter, more muted, warmer than Military Shade.
The same difference applies the other way round: Military Shade is a little darker, more saturated, cooler.
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 Military Shade on the colour wheel See Mortarion Grime on the colour wheel
All matches: Military Shade equivalents · Mortarion Grime equivalents