Fire Giant Orange vs Rust

How close are they?

ΔE 27.5, loose: different enough that you should pick on purpose. Rust is noticeably darker, much more muted, cooler than Fire Giant Orange.

Can I use Rust instead of Fire Giant Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Rust is noticeably darker, much more muted, cooler than Fire Giant Orange.

Can I use Fire Giant Orange instead of Rust?

The same difference applies the other way round: Fire Giant Orange is noticeably lighter, much more saturated, warmer.

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.