Amethyst Alchemy vs Crimson Magenta

How close are they?

ΔE 26.9, loose: different enough that you should pick on purpose. Crimson Magenta is much more saturated, warmer than Amethyst Alchemy.

Can I use Crimson Magenta instead of Amethyst Alchemy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Crimson Magenta is much more saturated, warmer than Amethyst Alchemy.

Can I use Amethyst Alchemy instead of Crimson Magenta?

The same difference applies the other way round: Amethyst Alchemy 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.