Lucius Lilac vs Light Violet

How close are they?

ΔE 16.0, loose: different enough that you should pick on purpose. Light Violet is noticeably darker, more saturated than Lucius Lilac.

Can I use Light Violet instead of Lucius Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Light Violet is noticeably darker, more saturated than Lucius Lilac.

Can I use Lucius Lilac instead of Light Violet?

The same difference applies the other way round: Lucius Lilac is noticeably lighter, 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.