ΔE 15.3, loose: different enough that you should pick on purpose. Castellan Green is a little lighter, more saturated, cooler than Forest Green (FS34079).
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Castellan Green is a little lighter, more saturated, cooler than Forest Green (FS34079).
The same difference applies the other way round: Forest Green (FS34079) is a little darker, more muted, 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 Forest Green (FS34079) on the colour wheel See Castellan Green on the colour wheel
All matches: Forest Green (FS34079) equivalents · Castellan Green equivalents