ΔE 8.9, close: similar, with a visible difference side by side. Castellan Green is more muted than Dark Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Castellan Green is more muted than Dark Green.
The same difference applies the other way round: Dark Green is 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 Dark Green on the colour wheel See Castellan Green on the colour wheel
All matches: Dark Green equivalents · Castellan Green equivalents