ΔE 38.5, loose: different enough that you should pick on purpose. Amethyst is noticeably darker, much more saturated, cooler than Light Magenta.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 39). Amethyst is noticeably darker, much more saturated, cooler than Light Magenta.
The same difference applies the other way round: Light Magenta is noticeably lighter, much 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 Light Magenta on the colour wheel See Amethyst on the colour wheel
All matches: Light Magenta equivalents · Amethyst equivalents