Diabolic Plum vs Dioxazine Purple

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Dioxazine Purple is noticeably darker, more muted than Diabolic Plum.

Can I use Dioxazine Purple instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Dioxazine Purple is noticeably darker, more muted than Diabolic Plum.

Can I use Diabolic Plum instead of Dioxazine Purple?

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