Mummified Grime vs Zombie

How close are they?

ΔE 16.9, loose: different enough that you should pick on purpose. Zombie is noticeably darker, cooler than Mummified Grime.

Can I use Zombie instead of Mummified Grime?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Zombie is noticeably darker, cooler than Mummified Grime.

Can I use Mummified Grime instead of Zombie?

The same difference applies the other way round: Mummified Grime is noticeably lighter, warmer.

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.