ΔE 14.1, fair: the same colour family, but not a straight swap. Ink Magenta is a little darker, warmer than Purple Alchemy. They are different kinds of paint: Army Painter Purple Alchemy is a one-coat contrast-style paint and Vallejo Ink Magenta is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Ink Magenta is a little darker, warmer than Purple Alchemy. They are different kinds of paint.
The same difference applies the other way round: Purple Alchemy is a little lighter, 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.
See Purple Alchemy on the colour wheel See Ink Magenta on the colour wheel
All matches: Purple Alchemy equivalents · Ink Magenta equivalents