ΔE 6.1, close: similar, with a visible difference side by side. Castellan Green is a little darker than Forest Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Castellan Green is a little darker than Forest Green.
The same difference applies the other way round: Forest Green is a little lighter.
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 Forest Green on the colour wheel See Castellan Green on the colour wheel
All matches: Forest Green equivalents · Castellan Green equivalents