Fairy Dust vs Sparkling Amethyst

How close are they?

ΔE 22.4, loose: different enough that you should pick on purpose. Sparkling Amethyst is noticeably darker, much more saturated than Fairy Dust.

Can I use Sparkling Amethyst instead of Fairy Dust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Sparkling Amethyst is noticeably darker, much more saturated than Fairy Dust.

Can I use Fairy Dust instead of Sparkling Amethyst?

The same difference applies the other way round: Fairy Dust is noticeably lighter, much more muted.

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.