Dark Green vs Castellan Green

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Castellan Green is a little lighter, warmer than Dark Green.

Can I use Castellan Green instead of Dark Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Castellan Green is a little lighter, warmer than Dark Green.

Can I use Dark Green instead of Castellan Green?

The same difference applies the other way round: Dark Green is a little darker, 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.