ΔE 12.1, fair: the same colour family, but not a straight swap. Cultist Purple is a little darker, more saturated than Matt Purple. They are different kinds of paint: AMMO by Mig Matt Purple is an ink, glaze or transparent paint and Army Painter Cultist Purple is an opaque base/layer 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). Cultist Purple is a little darker, more saturated than Matt Purple. They are different kinds of paint.
The same difference applies the other way round: Matt Purple is a little lighter, more muted.
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 Matt Purple on the colour wheel See Cultist Purple on the colour wheel
All matches: Matt Purple equivalents · Cultist Purple equivalents