Mecha Dark Green vs Castellan Green

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Castellan Green is a little lighter, more saturated, warmer than Mecha Dark Green.

Can I use Castellan Green instead of Mecha Dark Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Castellan Green is a little lighter, more saturated, warmer than Mecha Dark Green.

Can I use Mecha Dark Green instead of Castellan Green?

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