Castellan Green vs Dragon Blue

How close are they?

ΔE 20.4, loose: different enough that you should pick on purpose. Dragon Blue is noticeably lighter, more muted, cooler than Castellan Green.

Can I use Dragon Blue instead of Castellan Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Dragon Blue is noticeably lighter, more muted, cooler than Castellan Green.

Can I use Castellan Green instead of Dragon Blue?

The same difference applies the other way round: Castellan Green is noticeably darker, more saturated, 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.