ΔE 20.6, loose: different enough that you should pick on purpose. Fluor Violet is noticeably lighter, much more muted than Kakophoni Purple. They are different kinds of paint: Citadel Kakophoni Purple is an opaque base/layer paint and Green Stuff World Fluor Violet 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). Fluor Violet is noticeably lighter, much more muted than Kakophoni Purple. They are different kinds of paint.
The same difference applies the other way round: Kakophoni Purple is noticeably darker, much more saturated.
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 Kakophoni Purple on the colour wheel See Fluor Violet on the colour wheel
All matches: Kakophoni Purple equivalents · Fluor Violet equivalents