Lucius Lilac vs Dark Sea Gray

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Dark Sea Gray is a little darker, much more muted than Lucius Lilac.

Can I use Dark Sea Gray instead of Lucius Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Sea Gray is a little darker, much more muted than Lucius Lilac.

Can I use Lucius Lilac instead of Dark Sea Gray?

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.