ΔE 34.2, loose: different enough that you should pick on purpose. Ancient Sepia is noticeably lighter, much more saturated, warmer than Mortarion Grime.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Ancient Sepia is noticeably lighter, much more saturated, warmer than Mortarion Grime.
The same difference applies the other way round: Mortarion Grime is noticeably darker, much more muted, 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 Mortarion Grime on the colour wheel See Ancient Sepia on the colour wheel
All matches: Mortarion Grime equivalents · Ancient Sepia equivalents