ΔE 15.1, loose: different enough that you should pick on purpose. Heavy Metal is noticeably lighter than Iron Warriors.
Can I use Heavy Metal instead of Iron Warriors?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Heavy Metal is noticeably lighter than Iron Warriors.
Can I use Iron Warriors instead of Heavy Metal?
The same difference applies the other way round: Iron Warriors is noticeably darker.
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.