ΔE 24.8, loose: different enough that you should pick on purpose. Muddy Wash is noticeably lighter, more saturated than Grimdark Shadow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Muddy 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 Muddy Wash on the colour wheel
All matches: Grimdark Shadow equivalents · Muddy Wash equivalents