ΔE 19.5, loose: different enough that you should pick on purpose. Runefang Steel is noticeably darker than Light Metal.
Can I use Runefang Steel instead of Light Metal?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Runefang Steel is noticeably darker than Light Metal.
Can I use Light Metal instead of Runefang Steel?
The same difference applies the other way round: Light Metal 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.