Lilac vs Light Pink

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Light Pink is a little darker, more saturated, warmer than Lilac.

Can I use Light Pink instead of Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Light Pink is a little darker, more saturated, warmer than Lilac.

Can I use Lilac instead of Light Pink?

The same difference applies the other way round: Lilac is a little lighter, more muted, cooler.

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.