Lilac vs Lucius Lilac

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. Lucius Lilac is much more muted, cooler than Lilac.

Can I use Lucius Lilac instead of Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Lucius Lilac is much more muted, cooler than Lilac.

Can I use Lilac instead of Lucius Lilac?

The same difference applies the other way round: Lilac 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.