ΔE 19.7, loose: different enough that you should pick on purpose. Phalanx Yellow is a little darker, more muted, warmer than Neon Yellow. They are different kinds of paint: Army Painter Neon Yellow is a fluorescent or effect paint and Citadel Phalanx Yellow is an opaque base/layer 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 20). Phalanx Yellow is a little darker, more muted, warmer than Neon Yellow. They are different kinds of paint.
The same difference applies the other way round: Neon Yellow is a little lighter, more saturated, 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 Neon Yellow on the colour wheel See Phalanx Yellow on the colour wheel
All matches: Neon Yellow equivalents · Phalanx Yellow equivalents