Dark Iron vs Rusty Metal

How close are they?

ΔE 18.8, loose: different enough that you should pick on purpose. Rusty Metal is noticeably lighter, more saturated than Dark Iron.

Can I use Rusty Metal instead of Dark Iron?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Rusty Metal is noticeably lighter, more saturated than Dark Iron.

Can I use Dark Iron instead of Rusty Metal?

The same difference applies the other way round: Dark 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.