ΔE 14.9, fair: the same colour family, but not a straight swap. Sepia Wash is noticeably lighter, warmer than Mortarion Grime.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Sepia Wash is noticeably lighter, warmer than Mortarion Grime.
The same difference applies the other way round: Mortarion Grime is noticeably darker, 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 Sepia Wash on the colour wheel
All matches: Mortarion Grime equivalents · Sepia Wash equivalents