Lucius Lilac vs Sky Grey

How close are they?

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

Can I use Sky Grey instead of Lucius Lilac?

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

Can I use Lucius Lilac instead of Sky Grey?

The same difference applies the other way round: Lucius Lilac is a little darker, 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.