ΔE 15.3, loose: different enough that you should pick on purpose. Fyreslayer Flesh is noticeably lighter, more muted, cooler than Grim Rust. They are different kinds of paint: Army Painter Grim Rust is a fluorescent or effect paint and Citadel Fyreslayer Flesh 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). Fyreslayer Flesh is noticeably lighter, more muted, cooler than Grim Rust. They are different kinds of paint.
The same difference applies the other way round: Grim Rust is noticeably darker, 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 Fyreslayer Flesh on the colour wheel
All matches: Grim Rust equivalents · Fyreslayer Flesh equivalents