ΔE 21.5, loose: different enough that you should pick on purpose. USAF Brown is noticeably darker, warmer than Viridian Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). USAF Brown is noticeably darker, warmer than Viridian Green.
The same difference applies the other way round: Viridian Green is noticeably 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 Viridian Green on the colour wheel See USAF Brown on the colour wheel
All matches: Viridian Green equivalents · USAF Brown equivalents