ΔE 12.5, fair: the same colour family, but not a straight swap. Castellan Green is more saturated, cooler than IJA #21 Midori iro (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 IJA #21 Midori iro (Green).
The same difference applies the other way round: IJA #21 Midori iro (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 IJA #21 Midori iro (Green) on the colour wheel See Castellan Green on the colour wheel
All matches: IJA #21 Midori iro (Green) equivalents · Castellan Green equivalents