ΔE 20.8, loose: different enough that you should pick on purpose. Green Rust is noticeably lighter, cooler than Nurgling Green. They are different kinds of paint: Citadel Nurgling Green is an opaque base/layer 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 21). Green Rust is noticeably lighter, cooler than Nurgling Green. They are different kinds of paint.
The same difference applies the other way round: Nurgling Green is noticeably darker, 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 Nurgling Green on the colour wheel See Green Rust on the colour wheel
All matches: Nurgling Green equivalents · Green Rust equivalents