Grimdark Shadow vs Agrax Earthshade

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Agrax Earthshade is more saturated than Grimdark Shadow.

Can I use Agrax Earthshade instead of Grimdark Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Agrax Earthshade is more saturated than Grimdark Shadow.

Can I use Grimdark Shadow instead of Agrax Earthshade?

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