Dark Emerald vs Metal Black

How close are they?

ΔE 29.4, loose: different enough that you should pick on purpose. Metal Black is noticeably darker, much more muted than Dark Emerald.

Can I use Metal Black instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Metal Black is noticeably darker, much more muted than Dark Emerald.

Can I use Dark Emerald instead of Metal Black?

The same difference applies the other way round: Dark Emerald is noticeably lighter, 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.