Plum vs Deep Magenta

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Deep Magenta is noticeably darker, warmer than Plum.

Can I use Deep Magenta instead of Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Deep Magenta is noticeably darker, warmer than Plum.

Can I use Plum instead of Deep Magenta?

The same difference applies the other way round: Plum is noticeably lighter, cooler.

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.