ΔE 17.0, loose: different enough that you should pick on purpose. Blue Ink is more saturated, cooler than Magos Purple. They are different kinds of paint: Citadel Magos Purple is a one-coat contrast-style paint and Formula P3 Blue Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Blue Ink is more saturated, cooler than Magos Purple. They are different kinds of paint.
The same difference applies the other way round: Magos Purple is 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.
See Magos Purple on the colour wheel See Blue Ink on the colour wheel
All matches: Magos Purple equivalents · Blue Ink equivalents