ΔE 13.5, fair: the same colour family, but not a straight swap. Amethyst Purple is a little darker, more saturated than Magos Purple. They are different kinds of paint: Citadel Magos 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 13). Amethyst Purple is a little darker, more saturated 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.
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 Amethyst Purple on the colour wheel
All matches: Magos Purple equivalents · Amethyst Purple equivalents