ΔE 6.7, close: similar, with a visible difference side by side. Grey Brown is a little lighter, warmer than US Dark Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Grey Brown is a little lighter, warmer than US Dark Green.
The same difference applies the other way round: US Dark Green is a little darker, 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 US Dark Green on the colour wheel See Grey Brown on the colour wheel
All matches: US Dark Green equivalents · Grey Brown equivalents