Rust vs Cardinal Purple

How close are they?

ΔE 25.3, loose: different enough that you should pick on purpose. Cardinal Purple is more saturated, cooler than Rust.

Can I use Cardinal Purple instead of Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Cardinal Purple is more saturated, cooler than Rust.

Can I use Rust instead of Cardinal Purple?

The same difference applies the other way round: Rust is more muted, 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.