ΔE 17.7, loose: different enough that you should pick on purpose. Fern Green is a little darker, much more muted than Jenkins Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Fern Green is a little darker, much more muted than Jenkins Green.
The same difference applies the other way round: Jenkins Green is a little lighter, much more saturated.
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 Jenkins Green on the colour wheel See Fern Green on the colour wheel
All matches: Jenkins Green equivalents · Fern Green equivalents