Magenta vs Witch Purple

How close are they?

ΔE 39.2, loose: different enough that you should pick on purpose. Witch Purple is noticeably lighter, more saturated, cooler than Magenta.

Can I use Witch Purple instead of Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 39). Witch Purple is noticeably lighter, more saturated, cooler than Magenta.

Can I use Magenta instead of Witch Purple?

The same difference applies the other way round: Magenta is noticeably darker, 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.