ΔE 11.5, fair: the same colour family, but not a straight swap. Fern Green is a little lighter, warmer than Dark Green.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Fern Green is a little lighter, warmer than Dark Green.
The same difference applies the other way round: 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 Dark Green on the colour wheel See Fern Green on the colour wheel
All matches: Dark Green equivalents · Fern Green equivalents