ΔE 17.0, loose: different enough that you should pick on purpose. Piggy Purple Ink is a little darker, much more muted, warmer than Periwinkle Purple. They are different kinds of paint: Army Painter Periwinkle Purple is a one-coat contrast-style 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.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Piggy Purple Ink is a little darker, much more muted, warmer than Periwinkle Purple. They are different kinds of paint.
The same difference applies the other way round: Periwinkle Purple is a little lighter, much 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 Periwinkle Purple on the colour wheel See Piggy Purple Ink on the colour wheel
All matches: Periwinkle Purple equivalents · Piggy Purple Ink equivalents