Metallic Blue vs Amethyst Purple

How close are they?

ΔE 43.1, loose: different enough that you should pick on purpose. Amethyst Purple is noticeably darker, more saturated, warmer than Metallic Blue.

Can I use Amethyst Purple instead of Metallic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 43). Amethyst Purple is noticeably darker, more saturated, warmer than Metallic Blue.

Can I use Metallic Blue instead of Amethyst Purple?

The same difference applies the other way round: Metallic Blue is noticeably lighter, 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.