Lavender vs Xereus Purple

How close are they?

ΔE 29.0, loose: different enough that you should pick on purpose. Xereus Purple is noticeably darker, much more saturated, warmer than Lavender.

Can I use Xereus Purple instead of Lavender?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Xereus Purple is noticeably darker, much more saturated, warmer than Lavender.

Can I use Lavender instead of Xereus Purple?

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