Mephiston Red vs Rust Orange

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Rust Orange is noticeably lighter, more muted than Mephiston Red.

Can I use Rust Orange instead of Mephiston Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Rust Orange is noticeably lighter, more muted than Mephiston Red.

Can I use Mephiston Red instead of Rust Orange?

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

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.