Diabolic Plum vs Permanent Violet Dark

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Permanent Violet Dark is much more saturated, warmer than Diabolic Plum.

Can I use Permanent Violet Dark instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Permanent Violet Dark is much more saturated, warmer than Diabolic Plum.

Can I use Diabolic Plum instead of Permanent Violet Dark?

The same difference applies the other way round: Diabolic Plum is much more muted, 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.