Iron Fist vs Obsidian Black

How close are they?

ΔE 32.4, loose: different enough that you should pick on purpose. Obsidian Black is noticeably darker, more saturated than Iron Fist.

Can I use Obsidian Black instead of Iron Fist?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Obsidian Black is noticeably darker, more saturated than Iron Fist.

Can I use Iron Fist instead of Obsidian Black?

The same difference applies the other way round: Iron Fist is noticeably lighter, more muted.

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.