ΔE 21.0, loose: different enough that you should pick on purpose. Light Ultramarine is a little darker, much more saturated, cooler than Alien Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Light Ultramarine is a little darker, much more saturated, cooler than Alien Purple.
The same difference applies the other way round: Alien Purple is a little lighter, 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 Alien Purple on the colour wheel See Light Ultramarine on the colour wheel
All matches: Alien Purple equivalents · Light Ultramarine equivalents