Dechala Lilac vs Pastel Violet

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Pastel Violet is noticeably darker, much more saturated than Dechala Lilac.

Can I use Pastel Violet instead of Dechala Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Pastel Violet is noticeably darker, much more saturated than Dechala Lilac.

Can I use Dechala Lilac instead of Pastel Violet?

The same difference applies the other way round: Dechala Lilac is noticeably lighter, much more muted.

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.