Diabolic Plum vs Arianrhod Blue

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Arianrhod Blue is a little lighter, cooler than Diabolic Plum.

Can I use Arianrhod Blue instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Arianrhod Blue is a little lighter, cooler than Diabolic Plum.

Can I use Diabolic Plum instead of Arianrhod Blue?

The same difference applies the other way round: Diabolic Plum is a little 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.