Alpha Blue vs Amethyst Purple

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Amethyst Purple is more muted, warmer than Alpha Blue.

Can I use Amethyst Purple instead of Alpha Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Amethyst Purple is more muted, warmer than Alpha Blue.

Can I use Alpha Blue instead of Amethyst Purple?

The same difference applies the other way round: Alpha Blue is more saturated, 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.