ΔE 10.8, fair: the same colour family, but not a straight swap. Steel is noticeably darker than Runefang Steel.
Can I use Steel instead of Runefang Steel?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Steel is noticeably darker than Runefang Steel.
Can I use Runefang Steel instead of Steel?
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.