Lilac vs Deep Sky Blue

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Deep Sky Blue is noticeably darker, cooler than Lilac.

Can I use Deep Sky Blue instead of Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Deep Sky Blue is noticeably darker, cooler than Lilac.

Can I use Lilac instead of Deep Sky Blue?

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