Grim Black vs Dark Green Brown

How close are they?

ΔE 18.7, loose: different enough that you should pick on purpose. Dark Green Brown is noticeably lighter, much more saturated than Grim Black.

Can I use Dark Green Brown instead of Grim Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Green Brown is noticeably lighter, much more saturated than Grim Black.

Can I use Grim Black instead of Dark Green Brown?

The same difference applies the other way round: Grim Black is noticeably darker, 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.