Gore-Grunta Fur vs Rust

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Rust is a little darker, much more saturated than Gore-Grunta Fur.

Can I use Rust instead of Gore-Grunta Fur?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Rust is a little darker, much more saturated than Gore-Grunta Fur.

Can I use Gore-Grunta Fur instead of Rust?

The same difference applies the other way round: Gore-Grunta Fur is a little 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.