ΔE 5.7, very close: most people won’t see a difference side by side. Green Ink is more saturated than Castellan Green. They are different kinds of paint: Citadel Castellan Green is an opaque base/layer paint and Reaper Green Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Yes, for most jobs. Very close match (ΔE 6). Green Ink is more saturated than Castellan Green. They are different kinds of paint.
The same difference applies the other way round: Castellan Green is 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 Castellan Green on the colour wheel See Green Ink on the colour wheel
All matches: Castellan Green equivalents · Green Ink equivalents