Grimdark Shadow vs Atramentum Black

How close are they?

ΔE 8.6, close: similar, with a visible difference side by side. Our swatches for the two are almost the same.

Can I use Atramentum Black instead of Grimdark Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). The swatches are almost the same.

Can I use Grimdark Shadow instead of Atramentum Black?

The same difference applies the other way round.

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.