ΔE 38.4, loose: different enough that you should pick on purpose. Steel (Metallic) is noticeably lighter than Adamantium Black.
Can I use Steel (Metallic) instead of Adamantium Black?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Steel (Metallic) is noticeably lighter than Adamantium Black.
Can I use Adamantium Black instead of Steel (Metallic)?
The same difference applies the other way round: Adamantium Black 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.