ΔE 14.2, fair: the same colour family, but not a straight swap. Castellan Green is a little lighter, more saturated, cooler than Dark Green (FS34102).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Castellan Green is a little lighter, more saturated, cooler than Dark Green (FS34102).
The same difference applies the other way round: Dark Green (FS34102) 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 Dark Green (FS34102) on the colour wheel See Castellan Green on the colour wheel
All matches: Dark Green (FS34102) equivalents · Castellan Green equivalents