ΔE 18.7, loose: different enough that you should pick on purpose. Smalt Hue is noticeably darker, much more saturated, cooler than Lucius Lilac.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Smalt Hue is noticeably darker, much more saturated, cooler than Lucius Lilac.
The same difference applies the other way round: Lucius Lilac is noticeably lighter, much more muted, 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.
See Lucius Lilac on the colour wheel See Smalt Hue on the colour wheel
All matches: Lucius Lilac equivalents · Smalt Hue equivalents