Blue Violet vs Xereus Purple

How close are they?

ΔE 26.5, loose: different enough that you should pick on purpose. Xereus Purple is noticeably darker, more muted than Blue Violet.

Can I use Xereus Purple instead of Blue Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Xereus Purple is noticeably darker, more muted than Blue Violet.

Can I use Blue Violet instead of Xereus Purple?

The same difference applies the other way round: Blue Violet is noticeably lighter, more saturated.

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.