ΔE 16.8, loose: different enough that you should pick on purpose. Park Green Flat is a little lighter, much more muted, cooler than Fern Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Park Green Flat is a little lighter, much more muted, cooler than Fern Green.
The same difference applies the other way round: Fern Green is a little darker, much more saturated, 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 Fern Green on the colour wheel See Park Green Flat on the colour wheel
All matches: Fern Green equivalents · Park Green Flat equivalents