Rust Tone vs Brown Grey Wash

How close are they?

ΔE 33.8, loose: different enough that you should pick on purpose. Brown Grey Wash is a little lighter, much more muted than Rust Tone.

Can I use Brown Grey Wash instead of Rust Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Brown Grey Wash is a little lighter, much more muted than Rust Tone.

Can I use Rust Tone instead of Brown Grey Wash?

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