ΔE 6.2, close: similar, with a visible difference side by side. Viridian Grim is a little lighter, cooler than Midnight Olive.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Viridian Grim is a little lighter, cooler than Midnight Olive.
The same difference applies the other way round: Midnight Olive is a little 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.
See Midnight Olive on the colour wheel See Viridian Grim on the colour wheel
All matches: Midnight Olive equivalents · Viridian Grim equivalents