Forest Green vs Fern Green

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Fern Green is noticeably lighter, more muted, warmer than Forest Green.

Can I use Fern Green instead of Forest Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Fern Green is noticeably lighter, more muted, warmer than Forest Green.

Can I use Forest Green instead of Fern Green?

The same difference applies the other way round: Forest Green is noticeably darker, more saturated, 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.