Plum vs Imperial Purple

How close are they?

ΔE 19.8, loose: different enough that you should pick on purpose. Imperial Purple is much more saturated, cooler than Plum.

Can I use Imperial Purple instead of Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Imperial Purple is much more saturated, cooler than Plum.

Can I use Plum instead of Imperial Purple?

The same difference applies the other way round: 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.