Ultramarine Violet vs Alien Purple

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Alien Purple is a little darker, more muted than Ultramarine Violet.

Can I use Alien Purple instead of Ultramarine Violet?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Alien Purple is a little darker, more muted than Ultramarine Violet.

Can I use Ultramarine Violet instead of Alien Purple?

The same difference applies the other way round: Ultramarine Violet is a little lighter, more saturated.

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.