Dark Plum vs Deep Magenta

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Deep Magenta is noticeably lighter, much more saturated than Dark Plum.

Can I use Deep Magenta instead of Dark Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Deep Magenta is noticeably lighter, much more saturated than Dark Plum.

Can I use Dark Plum instead of Deep Magenta?

The same difference applies the other way round: Dark Plum is noticeably darker, much more muted.

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.