Pixie Pink vs Permanent Violet Dark

How close are they?

ΔE 35.7, loose: different enough that you should pick on purpose. Permanent Violet Dark is noticeably darker, cooler than Pixie Pink.

Can I use Permanent Violet Dark instead of Pixie Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Permanent Violet Dark is noticeably darker, cooler than Pixie Pink.

Can I use Pixie Pink instead of Permanent Violet Dark?

The same difference applies the other way round: Pixie Pink is noticeably lighter, 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.