Deep Violet vs Lucius Lilac

How close are they?

ΔE 24.3, loose: different enough that you should pick on purpose. Lucius Lilac is a little lighter, much more muted, cooler than Deep Violet.

Can I use Lucius Lilac instead of Deep Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Lucius Lilac is a little lighter, much more muted, cooler than Deep Violet.

Can I use Deep Violet instead of Lucius Lilac?

The same difference applies the other way round: Deep Violet is a little darker, 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.