ΔE 15.5, loose: different enough that you should pick on purpose. Fern Green is a little lighter, much more saturated, warmer than Wyvern Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Fern Green is a little lighter, much more saturated, warmer than Wyvern Green.
The same difference applies the other way round: Wyvern Green is a little darker, much more muted, cooler.
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 Wyvern Green on the colour wheel See Fern Green on the colour wheel
All matches: Wyvern Green equivalents · Fern Green equivalents