ΔE 9.7, close: similar, with a visible difference side by side. Castellan Green is a little darker, more saturated, warmer than Phtalo Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Castellan Green is a little darker, more saturated, warmer than Phtalo Green.
The same difference applies the other way round: Phtalo Green is a little lighter, more muted, cooler.
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 Phtalo Green on the colour wheel See Castellan Green on the colour wheel
All matches: Phtalo Green equivalents · Castellan Green equivalents