Grimdark Shadow vs Brown Wash

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Brown Wash is a little darker, much more saturated than Grimdark Shadow.

Can I use Brown Wash instead of Grimdark Shadow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Brown Wash is a little darker, much more saturated than Grimdark Shadow.

Can I use Grimdark Shadow instead of Brown Wash?

The same difference applies the other way round: Grimdark Shadow is a little lighter, much 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.