ΔE 53.9, loose: different enough that you should pick on purpose. Heavy Armour is noticeably darker than Runefang Steel.
Can I use Heavy Armour instead of Runefang Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 54). Heavy Armour is noticeably darker than Runefang Steel.
Can I use Runefang Steel instead of Heavy Armour?
The same difference applies the other way round: Runefang Steel 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.