ΔE 11.5, fair: the same colour family, but not a straight swap. Fern Green is a little lighter, warmer than Black 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 Black Green.
The same difference applies the other way round: Black 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 Black Green on the colour wheel See Fern Green on the colour wheel
All matches: Black Green equivalents · Fern Green equivalents