ΔE 7.9, close: similar, with a visible difference side by side. Fern Green is a little lighter, more saturated, cooler than Dark Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Fern Green is a little lighter, more saturated, cooler than Dark Green.
The same difference applies the other way round: Dark Green is a little darker, more muted, 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 Dark Green on the colour wheel See Fern Green on the colour wheel
All matches: Dark Green equivalents · Fern Green equivalents