Fey Pink vs Lucius Lilac

How close are they?

ΔE 24.6, loose: different enough that you should pick on purpose. Lucius Lilac is much more muted, cooler than Fey Pink.

Can I use Lucius Lilac instead of Fey Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Lucius Lilac is much more muted, cooler than Fey Pink.

Can I use Fey Pink instead of Lucius Lilac?

The same difference applies the other way round: Fey Pink is much 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.