ΔE 7.8, close: similar, with a visible difference side by side. Grey Brown is a little darker, warmer than Olive Drab.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Grey Brown is a little darker, warmer than Olive Drab.
The same difference applies the other way round: Olive Drab is a little lighter, cooler.
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 Olive Drab on the colour wheel See Grey Brown on the colour wheel
All matches: Olive Drab equivalents · Grey Brown equivalents