ΔE 15.8, loose: different enough that you should pick on purpose. Atramentum Black is more muted than Agrax Earthshade.
Can I use Atramentum Black instead of Agrax Earthshade?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Atramentum Black is more muted than Agrax Earthshade.
Can I use Agrax Earthshade instead of Atramentum Black?
The same difference applies the other way round: Agrax Earthshade is more saturated.
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.