ΔE 10.1, fair: the same colour family, but not a straight swap. Obsidian Black is a little lighter than Ironbreaker.
Can I use Obsidian Black instead of Ironbreaker?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Obsidian Black is a little lighter than Ironbreaker.
Can I use Ironbreaker instead of Obsidian Black?
The same difference applies the other way round: Ironbreaker is a little 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.