ΔE 21.7, loose: different enough that you should pick on purpose. Fluor Magenta is a little lighter, much more saturated than Pink Horror. They are different kinds of paint: Citadel Pink Horror is an opaque base/layer paint and Green Stuff World Fluor Magenta is an ink, glaze or transparent 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 22). Fluor Magenta is a little lighter, much more saturated than Pink Horror. They are different kinds of paint.
The same difference applies the other way round: Pink Horror is a little darker, much more muted.
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 Pink Horror on the colour wheel See Fluor Magenta on the colour wheel
All matches: Pink Horror equivalents · Fluor Magenta equivalents