Rust Orange vs Flat Red

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Flat Red is noticeably darker, more saturated, cooler than Rust Orange.

Can I use Flat Red instead of Rust Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Flat Red is noticeably darker, more saturated, cooler than Rust Orange.

Can I use Rust Orange instead of Flat Red?

The same difference applies the other way round: Rust Orange is noticeably lighter, 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.