ΔE 13.1, fair: the same colour family, but not a straight swap. Castellan Green is a little lighter, more saturated than Protective Green 1920s-1930s.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Castellan Green is a little lighter, more saturated than Protective Green 1920s-1930s.
The same difference applies the other way round: Protective Green 1920s-1930s is a little darker, more muted.
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 Protective Green 1920s-1930s on the colour wheel See Castellan Green on the colour wheel
All matches: Protective Green 1920s-1930s equivalents · Castellan Green equivalents