Steel Red vs Obsidian Black

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Obsidian Black is a little lighter, more muted than Steel Red.

Can I use Obsidian Black instead of Steel Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Obsidian Black is a little lighter, more muted than Steel Red.

Can I use Steel Red instead of Obsidian Black?

The same difference applies the other way round: Steel Red is a little darker, 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.