ΔE 7.3, close: similar, with a visible difference side by side. Fern Green is a little darker, more saturated than Olive Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Fern Green is a little darker, more saturated than Olive Green.
The same difference applies the other way round: Olive Green is a little lighter, 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 Olive Green on the colour wheel See Fern Green on the colour wheel
All matches: Olive Green equivalents · Fern Green equivalents