ΔE 10.3, fair: the same colour family, but not a straight swap. Flat Green is a little lighter, cooler than Forest Green (FS34079).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Flat Green is a little lighter, cooler than Forest Green (FS34079).
The same difference applies the other way round: Forest Green (FS34079) 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 Forest Green (FS34079) on the colour wheel See Flat Green on the colour wheel
All matches: Forest Green (FS34079) equivalents · Flat Green equivalents