ΔE 15.0, loose: different enough that you should pick on purpose. Gore-Grunta Fur is more muted, cooler than Grim Rust. They are different kinds of paint: Army Painter Grim Rust is a fluorescent or effect paint and Citadel Gore-Grunta Fur is a one-coat contrast-style paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Gore-Grunta Fur is more muted, cooler than Grim Rust. They are different kinds of paint.
The same difference applies the other way round: Grim Rust is 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.
See Grim Rust on the colour wheel See Gore-Grunta Fur on the colour wheel
All matches: Grim Rust equivalents · Gore-Grunta Fur equivalents