ΔE 13.5, fair: the same colour family, but not a straight swap. Heavy Metal is noticeably darker than Ironbreaker.
Can I use Heavy Metal instead of Ironbreaker?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Heavy Metal is noticeably darker than Ironbreaker.
Can I use Ironbreaker instead of Heavy Metal?
The same difference applies the other way round: Ironbreaker is noticeably lighter.
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.