Castellan Green vs Field Green-Grey

How close are they?

ΔE 7.2, close: similar, with a visible difference side by side. Field Green-Grey is a little darker, more saturated than Castellan Green.

Can I use Field Green-Grey instead of Castellan Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Field Green-Grey is a little darker, more saturated than Castellan Green.

Can I use Castellan Green instead of Field Green-Grey?

The same difference applies the other way round: Castellan Green is a little lighter, 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.