ΔE 11.8, fair: the same colour family, but not a straight swap. Amethyst is a little lighter, cooler than Medium Magenta.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Amethyst is a little lighter, cooler than Medium Magenta.
The same difference applies the other way round: Medium Magenta is a little darker, 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 Medium Magenta on the colour wheel See Amethyst on the colour wheel
All matches: Medium Magenta equivalents · Amethyst equivalents