ΔE 15.6, loose: different enough that you should pick on purpose. Fluorescent Green is a little lighter, cooler than Warpstone Glow. They are different kinds of paint: Citadel Warpstone Glow is an opaque base/layer paint and Mr. Hobby 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 16). Fluorescent Green is a little lighter, cooler than Warpstone Glow. They are different kinds of paint.
The same difference applies the other way round: Warpstone Glow is a little 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 Warpstone Glow on the colour wheel See Fluorescent Green on the colour wheel
All matches: Warpstone Glow equivalents · Fluorescent Green equivalents