ΔE 14.3, fair: the same colour family, but not a straight swap. Obsidian Black is noticeably lighter than Plate Armour.
Can I use Obsidian Black instead of Plate Armour?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Obsidian Black is noticeably lighter than Plate Armour.
Can I use Plate Armour instead of Obsidian Black?
The same difference applies the other way round: Plate Armour 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.