ΔE 20.9, loose: different enough that you should pick on purpose. Fluorescent Yellow is a little lighter, more saturated, cooler than Phalanx Yellow. They are different kinds of paint: Citadel Phalanx Yellow is an opaque base/layer paint and Vallejo Fluorescent Yellow 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 Yellow is a little lighter, more saturated, cooler than Phalanx Yellow. They are different kinds of paint.
The same difference applies the other way round: Phalanx Yellow is a little darker, 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 Phalanx Yellow on the colour wheel See Fluorescent Yellow on the colour wheel
All matches: Phalanx Yellow equivalents · Fluorescent Yellow equivalents