Diabolic Plum vs Nightmare Purple

How close are they?

ΔE 9.3, close: similar, with a visible difference side by side. Nightmare Purple is a little darker, more muted than Diabolic Plum.

Can I use Nightmare Purple instead of Diabolic Plum?

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

Can I use Diabolic Plum instead of Nightmare Purple?

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