Dark Emerald vs Boiler Black

How close are they?

ΔE 25.4, loose: different enough that you should pick on purpose. Boiler Black is a little lighter, much more muted than Dark Emerald.

Can I use Boiler Black instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Boiler Black is a little lighter, much more muted than Dark Emerald.

Can I use Dark Emerald instead of Boiler Black?

The same difference applies the other way round: Dark Emerald is a little darker, much more saturated.

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.