Grim Black vs Zombie

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Zombie is noticeably lighter, more saturated than Grim Black.

Can I use Zombie instead of Grim Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Zombie is noticeably lighter, more saturated than Grim Black.

Can I use Grim Black instead of Zombie?

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