ΔE 10.8, fair: the same colour family, but not a straight swap. Flat Green is a little lighter, more muted, warmer than Forest Green.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Flat Green is a little lighter, more muted, warmer than Forest Green.
The same difference applies the other way round: Forest Green is a little darker, 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 Forest Green on the colour wheel See Flat Green on the colour wheel
All matches: Forest Green equivalents · Flat Green equivalents