Fey Pink vs Lilac Purple

How close are they?

ΔE 19.4, loose: different enough that you should pick on purpose. Lilac Purple is noticeably darker, cooler than Fey Pink.

Can I use Lilac Purple instead of Fey Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Lilac Purple is noticeably darker, cooler than Fey Pink.

Can I use Fey Pink instead of Lilac Purple?

The same difference applies the other way round: Fey 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.