Blue Violet vs Dechala Lilac

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Dechala Lilac is noticeably lighter, warmer than Blue Violet.

Can I use Dechala Lilac instead of Blue Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dechala Lilac is noticeably lighter, warmer than Blue Violet.

Can I use Blue Violet instead of Dechala Lilac?

The same difference applies the other way round: Blue Violet is noticeably darker, 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.