ΔE 17.4, loose: different enough that you should pick on purpose. Copper Brown is a little lighter, more saturated than Grim Black.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Copper Brown is a little lighter, more saturated than Grim Black.
The same difference applies the other way round: Grim Black is a little darker, 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.
See Grim Black on the colour wheel See Copper Brown on the colour wheel
All matches: Grim Black equivalents · Copper Brown equivalents