Deep Violet vs Diabolic Plum

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Diabolic Plum is much more saturated, cooler than Deep Violet.

Can I use Diabolic Plum instead of Deep Violet?

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

Can I use Deep Violet instead of Diabolic Plum?

The same difference applies the other way round: Deep Violet is much more muted, 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.