Dark Emerald vs Decayed Metal

How close are they?

ΔE 21.3, loose: different enough that you should pick on purpose. Decayed Metal is much more muted, warmer than Dark Emerald.

Can I use Decayed Metal instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Decayed Metal is much more muted, warmer than Dark Emerald.

Can I use Dark Emerald instead of Decayed Metal?

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