ΔE 23.1, loose: different enough that you should pick on purpose. Green Rust is noticeably lighter, much more muted than Hexwraith Flame. They are different kinds of paint: Citadel Hexwraith Flame is a one-coat contrast-style paint and Vallejo Green Rust is a fluorescent or effect 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 23). Green Rust is noticeably lighter, much more muted than Hexwraith Flame. They are different kinds of paint.
The same difference applies the other way round: Hexwraith Flame is noticeably 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.
See Hexwraith Flame on the colour wheel See Green Rust on the colour wheel
All matches: Hexwraith Flame equivalents · Green Rust equivalents