Lucius Lilac vs Grey Blue

How close are they?

ΔE 19.4, loose: different enough that you should pick on purpose. Grey Blue is noticeably darker, more muted, cooler than Lucius Lilac.

Can I use Grey Blue instead of Lucius Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Grey Blue is noticeably darker, more muted, cooler than Lucius Lilac.

Can I use Lucius Lilac instead of Grey Blue?

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