ΔE 21.5, loose: different enough that you should pick on purpose. Cool Grey is a little darker, much more muted than Lucius Lilac.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Cool Grey is a little darker, much more muted than Lucius Lilac.
The same difference applies the other way round: Lucius Lilac is a little lighter, much more saturated.
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 Cool Grey on the colour wheel
All matches: Lucius Lilac equivalents · Cool Grey equivalents