ΔE 11.0, fair: the same colour family, but not a straight swap. Fluor Magenta is a little lighter than Warlock Purple. They are different kinds of paint: Army Painter Warlock Purple 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.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Fluor Magenta is a little lighter than Warlock Purple. They are different kinds of paint.
The same difference applies the other way round: Warlock Purple is a little darker.
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 Warlock Purple on the colour wheel See Fluor Magenta on the colour wheel
All matches: Warlock Purple equivalents · Fluor Magenta equivalents