ΔE 26.7, loose: different enough that you should pick on purpose. Cygnar Blue Highlight is much 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 Cygnar Blue Highlight is an opaque base/layer 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 27). Cygnar Blue Highlight is much more saturated, cooler than Magos Purple. They are different kinds of paint.
The same difference applies the other way round: Magos Purple is 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.
See Magos Purple on the colour wheel See Cygnar Blue Highlight on the colour wheel
All matches: Magos Purple equivalents · Cygnar Blue Highlight equivalents