Light Ultramarine Blue vs Alien Purple

How close are they?

ΔE 21.7, loose: different enough that you should pick on purpose. Alien Purple is noticeably darker, more muted, warmer than Light Ultramarine Blue.

Can I use Alien Purple instead of Light Ultramarine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Alien Purple is noticeably darker, more muted, warmer than Light Ultramarine Blue.

Can I use Light Ultramarine Blue instead of Alien Purple?

The same difference applies the other way round: Light Ultramarine Blue is noticeably lighter, 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.