Alien Purple vs Pastel Violet

How close are they?

ΔE 19.5, loose: different enough that you should pick on purpose. Pastel Violet is noticeably lighter, warmer than Alien Purple.

Can I use Pastel Violet instead of Alien Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Pastel Violet is noticeably lighter, warmer than Alien Purple.

Can I use Alien Purple instead of Pastel Violet?

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