ΔE 16.2, loose: different enough that you should pick on purpose. Arianrhod Blue is a little lighter, cooler than Xereus Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Arianrhod Blue is a little lighter, cooler than Xereus Purple.
The same difference applies the other way round: Xereus Purple is a little darker, 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.
See Xereus Purple on the colour wheel See Arianrhod Blue on the colour wheel
All matches: Xereus Purple equivalents · Arianrhod Blue equivalents