Heavy Metal vs Dark Iron

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Dark Iron is a little lighter, much more saturated than Heavy Metal.

Can I use Dark Iron instead of Heavy Metal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Dark Iron is a little lighter, much more saturated than Heavy Metal.

Can I use Heavy Metal instead of Dark Iron?

The same difference applies the other way round: Heavy Metal is a little darker, much 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.