Rust Tone vs Flesh Wash

How close are they?

ΔE 37.9, loose: different enough that you should pick on purpose. Flesh Wash is noticeably lighter, much more muted, cooler than Rust Tone.

Can I use Flesh Wash instead of Rust Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Flesh Wash is noticeably lighter, much more muted, cooler than Rust Tone.

Can I use Rust Tone instead of Flesh Wash?

The same difference applies the other way round: Rust Tone is noticeably darker, much 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.