Amethyst vs Blueberry

How close are they?

ΔE 24.4, loose: different enough that you should pick on purpose. Blueberry is a little lighter, more muted, cooler than Amethyst.

Can I use Blueberry instead of Amethyst?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Blueberry is a little lighter, more muted, cooler than Amethyst.

Can I use Amethyst instead of Blueberry?

The same difference applies the other way round: Amethyst is a little darker, more saturated, warmer.

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.