Diabolic Plum vs Amethyst Rayne

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Amethyst Rayne is a little lighter, much more muted, cooler than Diabolic Plum.

Can I use Amethyst Rayne instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Amethyst Rayne is a little lighter, much more muted, cooler than Diabolic Plum.

Can I use Diabolic Plum instead of Amethyst Rayne?

The same difference applies the other way round: Diabolic Plum is a little darker, 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.