ΔE 14.0, fair: the same colour family, but not a straight swap. Olive Green is much more muted than Castellan Green.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Olive Green is much more muted than Castellan Green.
The same difference applies the other way round: Castellan Green is much more saturated.
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 Castellan Green on the colour wheel See Olive Green on the colour wheel
All matches: Castellan Green equivalents · Olive Green equivalents