Guilliman Flesh vs Rust

How close are they?

ΔE 37.7, loose: different enough that you should pick on purpose. Rust is noticeably darker, much more saturated than Guilliman Flesh.

Can I use Rust instead of Guilliman Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Rust is noticeably darker, much more saturated than Guilliman Flesh.

Can I use Guilliman Flesh instead of Rust?

The same difference applies the other way round: Guilliman Flesh is noticeably lighter, much more muted.

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.