ΔE 14.2, fair: the same colour family, but not a straight swap. Obsidian Black is noticeably darker than Metallic Gray.
Can I use Obsidian Black instead of Metallic Gray?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Obsidian Black is noticeably darker than Metallic Gray.
Can I use Metallic Gray instead of Obsidian Black?
The same difference applies the other way round: Metallic Gray 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.