Dark Green Blue vs Monastic Green

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Monastic Green is a little darker, more saturated, warmer than Dark Green Blue.

Can I use Monastic Green instead of Dark Green Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Monastic Green is a little darker, more saturated, warmer than Dark Green Blue.

Can I use Dark Green Blue instead of Monastic Green?

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