Dark Emerald vs Iridescent Gold Deep (Fine)

How close are they?

ΔE 40.1, loose: different enough that you should pick on purpose. Iridescent Gold Deep (Fine) is noticeably lighter, more saturated, warmer than Dark Emerald. They are different kinds of paint: Army Painter Dark Emerald is a metallic and Golden Iridescent Gold Deep (Fine) is a pearl paint, so they won’t behave the same on the model even where the colour is close.

Can I use Iridescent Gold Deep (Fine) instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 40). Iridescent Gold Deep (Fine) is noticeably lighter, more saturated, warmer than Dark Emerald. They are different kinds of paint.

Can I use Dark Emerald instead of Iridescent Gold Deep (Fine)?

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