ΔE 21.2, loose: different enough that you should pick on purpose. Fluorescent Green is much more saturated, cooler than Warp Lightning. They are different kinds of paint: Citadel Warp Lightning is a one-coat contrast-style paint and Vallejo Fluorescent Green 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). Fluorescent Green is much more saturated, cooler than Warp Lightning. They are different kinds of paint.
The same difference applies the other way round: Warp Lightning is much more muted, 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 Warp Lightning on the colour wheel See Fluorescent Green on the colour wheel
All matches: Warp Lightning equivalents · Fluorescent Green equivalents