ΔE 13.5, fair: the same colour family, but not a straight swap. Rusty Metal is noticeably darker than Dark Steel.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Rusty Metal is noticeably darker than Dark Steel.
The same difference applies the other way round: Dark 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.
See Dark Steel on the colour wheel See Rusty Metal on the colour wheel
All matches: Dark Steel equivalents · Rusty Metal equivalents