Alien Purple vs Arianrhod Blue

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Arianrhod Blue is a little darker, cooler than Alien Purple.

Can I use Arianrhod Blue instead of Alien Purple?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Arianrhod Blue is a little darker, cooler than Alien Purple.

Can I use Alien Purple instead of Arianrhod Blue?

The same difference applies the other way round: Alien Purple is a little lighter, 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.