ΔE 23.8, loose: different enough that you should pick on purpose. Green Rust is noticeably lighter, warmer than Nihilakh Oxide. They are different kinds of paint: Citadel Nihilakh Oxide is a technical paint or medium 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 24). Green Rust is noticeably lighter, warmer than Nihilakh Oxide. They are different kinds of paint.
The same difference applies the other way round: Nihilakh Oxide is noticeably darker, cooler.
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 Nihilakh Oxide on the colour wheel See Green Rust on the colour wheel
All matches: Nihilakh Oxide equivalents · Green Rust equivalents