ΔE 16.3, loose: different enough that you should pick on purpose. Green Brown is noticeably darker, more muted, warmer than Greatcoat Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Green Brown is noticeably darker, more muted, warmer than Greatcoat Green.
The same difference applies the other way round: Greatcoat Green is noticeably lighter, more saturated, 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 Greatcoat Green on the colour wheel See Green Brown on the colour wheel
All matches: Greatcoat Green equivalents · Green Brown equivalents