Warlock Purple vs Alien Purple

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Alien Purple is noticeably lighter, cooler than Warlock Purple.

Can I use Alien Purple instead of Warlock Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Alien Purple is noticeably lighter, cooler than Warlock Purple.

Can I use Warlock Purple instead of Alien Purple?

The same difference applies the other way round: Warlock Purple is noticeably darker, 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.