ΔE 13.9, fair: the same colour family, but not a straight swap. Piggy Purple Ink is a little darker, more muted, warmer than Cultist Purple. They are different kinds of paint: Army Painter Cultist Purple is an opaque base/layer paint and Formula P3 Piggy Purple Ink 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 14). Piggy Purple Ink is a little darker, more muted, warmer than Cultist Purple. They are different kinds of paint.
The same difference applies the other way round: Cultist Purple 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 Cultist Purple on the colour wheel See Piggy Purple Ink on the colour wheel
All matches: Cultist Purple equivalents · Piggy Purple Ink equivalents