ΔE 15.8, loose: different enough that you should pick on purpose. Amethyst Purple is noticeably lighter than Kunzite Alchemy.
Can I use Amethyst Purple instead of Kunzite Alchemy?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Amethyst Purple is noticeably lighter than Kunzite Alchemy.
Can I use Kunzite Alchemy instead of Amethyst Purple?
The same difference applies the other way round: Kunzite Alchemy is noticeably darker.
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.