Bright Red vs Rust

How close are they?

ΔE 21.1, loose: different enough that you should pick on purpose. Rust is noticeably darker, more muted, warmer than Bright Red.

Can I use Rust instead of Bright Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Rust is noticeably darker, more muted, warmer than Bright Red.

Can I use Bright Red instead of Rust?

The same difference applies the other way round: Bright Red is noticeably lighter, 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.