ΔE 17.6, loose: different enough that you should pick on purpose. Lucius Lilac is a little lighter, warmer than RAF Azure Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Lucius Lilac is a little lighter, warmer than RAF Azure Blue.
The same difference applies the other way round: RAF Azure Blue is a little darker, cooler.
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 RAF Azure Blue on the colour wheel See Lucius Lilac on the colour wheel
All matches: RAF Azure Blue equivalents · Lucius Lilac equivalents