ΔE 12.8, fair: the same colour family, but not a straight swap. Amethyst Purple is noticeably lighter, more muted than Shyish Purple. They are different kinds of paint: Citadel Shyish 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 noticeably lighter, more muted than Shyish Purple. They are different kinds of paint.
The same difference applies the other way round: Shyish Purple is noticeably 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 Shyish Purple on the colour wheel See Amethyst Purple on the colour wheel
All matches: Shyish Purple equivalents · Amethyst Purple equivalents