ΔE 26.7, loose: different enough that you should pick on purpose. Sepia Wash is noticeably lighter, more saturated than Grimdark Shadow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Sepia Wash is noticeably lighter, more saturated than Grimdark Shadow.
The same difference applies the other way round: Grimdark Shadow is noticeably darker, 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 Sepia Wash on the colour wheel
All matches: Grimdark Shadow equivalents · Sepia Wash equivalents