ΔE 18.7, loose: different enough that you should pick on purpose. Rust (Metallic) is noticeably lighter than Runefang Steel.
Can I use Rust (Metallic) instead of Runefang Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Rust (Metallic) is noticeably lighter than Runefang Steel.
Can I use Runefang Steel instead of Rust (Metallic)?
The same difference applies the other way round: Runefang Steel 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.