Dark Olive Green vs Castellan Green

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Castellan Green is a little lighter, more saturated, cooler than Dark Olive Green.

Can I use Castellan Green instead of Dark Olive Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Castellan Green is a little lighter, more saturated, cooler than Dark Olive Green.

Can I use Dark Olive Green instead of Castellan Green?

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

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.