Pure Red vs Fire Orange

How close are they?

ΔE 27.3, loose: different enough that you should pick on purpose. Fire Orange is noticeably lighter, much more muted, warmer than Pure Red.

Can I use Fire Orange instead of Pure Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Fire Orange is noticeably lighter, much more muted, warmer than Pure Red.

Can I use Pure Red instead of Fire Orange?

The same difference applies the other way round: Pure Red is noticeably darker, much more saturated, cooler.

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.