ΔE 12.6, fair: the same colour family, but not a straight swap. Castellan Green is more saturated, cooler than WWI German Dark Green.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Castellan Green is more saturated, cooler than WWI German Dark Green.
The same difference applies the other way round: WWI German Dark Green is 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 WWI German Dark Green on the colour wheel See Castellan Green on the colour wheel
All matches: WWI German Dark Green equivalents · Castellan Green equivalents