Diabolic Plum vs Purple

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Purple is much more saturated, cooler than Diabolic Plum.

Can I use Purple instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Purple is much more saturated, cooler than Diabolic Plum.

Can I use Diabolic Plum instead of Purple?

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