Mortarion Grime vs Necrosis Green Wash

How close are they?

ΔE 24.0, loose: different enough that you should pick on purpose. Necrosis Green Wash is noticeably lighter, cooler than Mortarion Grime.

Can I use Necrosis Green Wash instead of Mortarion Grime?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Necrosis Green Wash is noticeably lighter, cooler than Mortarion Grime.

Can I use Mortarion Grime instead of Necrosis Green Wash?

The same difference applies the other way round: Mortarion Grime is noticeably darker, 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.