Dark Emerald vs Dark Iron

How close are they?

ΔE 23.9, loose: different enough that you should pick on purpose. Dark Iron is a little lighter, more muted, warmer than Dark Emerald.

Can I use Dark Iron instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Dark Iron is a little lighter, more muted, warmer than Dark Emerald.

Can I use Dark Emerald instead of Dark Iron?

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