ΔE 11.8, fair: the same colour family, but not a straight swap. Sacra Purple is a little lighter, more muted than Alien Purple. They are different kinds of paint: Army Painter Alien Purple is an opaque base/layer paint and Green Stuff World Sacra 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). Sacra Purple is a little lighter, more muted than Alien Purple. They are different kinds of paint.
The same difference applies the other way round: Alien Purple is a little darker, more saturated.
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 Alien Purple on the colour wheel See Sacra Purple on the colour wheel
All matches: Alien Purple equivalents · Sacra Purple equivalents