Burnt Iron vs Obsidian Black

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Obsidian Black is noticeably lighter, much more muted than Burnt Iron.

Can I use Obsidian Black instead of Burnt Iron?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Obsidian Black is noticeably lighter, much more muted than Burnt Iron.

Can I use Burnt Iron instead of Obsidian Black?

The same difference applies the other way round: Burnt Iron is noticeably darker, much more saturated.

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.