Lilac vs Blue Flux

How close are they?

ΔE 22.9, loose: different enough that you should pick on purpose. Blue Flux is a little darker, much more saturated, cooler than Lilac.

Can I use Blue Flux instead of Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Blue Flux is a little darker, much more saturated, cooler than Lilac.

Can I use Lilac instead of Blue Flux?

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