ΔE 12.4, fair: the same colour family, but not a straight swap. Permanent Violet Dark is a little darker, warmer than Xereus Purple. They are different kinds of paint: Citadel Xereus Purple is an opaque base/layer paint and Golden Permanent Violet Dark 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). Permanent Violet Dark is a little darker, warmer than Xereus Purple. They are different kinds of paint.
The same difference applies the other way round: Xereus Purple is a little lighter, 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 Xereus Purple on the colour wheel See Permanent Violet Dark on the colour wheel
All matches: Xereus Purple equivalents · Permanent Violet Dark equivalents