ΔE 11.7, fair: the same colour family, but not a straight swap. Amethyst Purple is a little lighter, warmer than Periwinkle Purple. They are different kinds of paint: Army Painter Periwinkle Purple is a one-coat contrast-style paint and Green Stuff World Amethyst Purple 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 12). Amethyst Purple is a little lighter, warmer than Periwinkle Purple. They are different kinds of paint.
The same difference applies the other way round: Periwinkle Purple is a little darker, 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 Amethyst Purple on the colour wheel
All matches: Periwinkle Purple equivalents · Amethyst Purple equivalents