ΔE 31.1, loose: different enough that you should pick on purpose. Aluminium (Metal.) is noticeably lighter than Tarnished Steel.
Can I use Aluminium (Metal.) instead of Tarnished Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Aluminium (Metal.) is noticeably lighter than Tarnished Steel.
Can I use Tarnished Steel instead of Aluminium (Metal.)?
The same difference applies the other way round: Tarnished 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.