ΔE 7.6, close: similar, with a visible difference side by side. Fern Green is a little lighter, more saturated than WWI German 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 than WWI German Dark Green.
The same difference applies the other way round: WWI German Dark Green is a little darker, more muted.
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 WWI German Dark Green on the colour wheel See Fern Green on the colour wheel
All matches: WWI German Dark Green equivalents · Fern Green equivalents