Diabolic Plum vs Deep Amethyst

How close are they?

ΔE 22.5, loose: different enough that you should pick on purpose. Deep Amethyst is a little darker, much more muted, cooler than Diabolic Plum.

Can I use Deep Amethyst instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Deep Amethyst is a little darker, much more muted, cooler than Diabolic Plum.

Can I use Diabolic Plum instead of Deep Amethyst?

The same difference applies the other way round: Diabolic Plum is a little lighter, much more saturated, 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.