ΔE 10.2, fair: the same colour family, but not a straight swap. Sacra Purple is a little darker, more saturated, cooler than Magos Purple. They are different kinds of paint: Citadel Magos Purple is a one-coat contrast-style 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 10). Sacra Purple is a little darker, more saturated, cooler than Magos Purple. They are different kinds of paint.
The same difference applies the other way round: Magos Purple is a little lighter, more muted, warmer.
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 Magos Purple on the colour wheel See Sacra Purple on the colour wheel
All matches: Magos Purple equivalents · Sacra Purple equivalents