Rough Iron vs Dark Iron

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Dark Iron is noticeably lighter, more saturated than Rough Iron.

Can I use Dark Iron instead of Rough Iron?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dark Iron is noticeably lighter, more saturated than Rough Iron.

Can I use Rough Iron instead of Dark Iron?

The same difference applies the other way round: Rough Iron is noticeably darker, more muted.

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.