Diabolic Plum vs Bad Bruise

How close are they?

ΔE 32.6, loose: different enough that you should pick on purpose. Bad Bruise is a little darker, much more muted than Diabolic Plum.

Can I use Bad Bruise instead of Diabolic Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Bad Bruise is a little darker, much more muted than Diabolic Plum.

Can I use Diabolic Plum instead of Bad Bruise?

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