ΔE 19.5, loose: different enough that you should pick on purpose. Alien Purple is much more muted, warmer than Neptune's Fury.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Alien Purple is much more muted, warmer than Neptune's Fury.
The same difference applies the other way round: Neptune's Fury is much more saturated, 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 Neptune's Fury on the colour wheel See Alien Purple on the colour wheel
All matches: Neptune's Fury equivalents · Alien Purple equivalents