Dark Rust vs Walnut Brown

How close are they?

ΔE 9.2, close: similar, with a visible difference side by side. Walnut Brown is a little darker, more saturated, cooler than Dark Rust.

Can I use Walnut Brown instead of Dark Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Walnut Brown is a little darker, more saturated, cooler than Dark Rust.

Can I use Dark Rust instead of Walnut Brown?

The same difference applies the other way round: Dark Rust is a little lighter, more muted, 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.