Gryph-Hound Orange vs Rust

How close are they?

ΔE 31.9, loose: different enough that you should pick on purpose. Rust is noticeably darker, more muted, cooler than Gryph-Hound Orange.

Can I use Rust instead of Gryph-Hound Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Rust is noticeably darker, more muted, cooler than Gryph-Hound Orange.

Can I use Gryph-Hound Orange instead of Rust?

The same difference applies the other way round: Gryph-Hound Orange is noticeably lighter, 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.