Grimdark Shadow vs Armor Wash

How close are they?

ΔE 7.7, close: similar, with a visible difference side by side. Armor Wash is a little lighter than Grimdark Shadow.

Can I use Armor Wash instead of Grimdark Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Armor Wash is a little lighter than Grimdark Shadow.

Can I use Grimdark Shadow instead of Armor Wash?

The same difference applies the other way round: Grimdark Shadow is a little darker.

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.