ΔE 16.6, loose: different enough that you should pick on purpose. Amethyst is much more saturated, warmer than Light Plum.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Amethyst is much more saturated, warmer than Light Plum.
The same difference applies the other way round: Light Plum is much more muted, cooler.
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 Plum on the colour wheel See Amethyst on the colour wheel
All matches: Light Plum equivalents · Amethyst equivalents