Fey Pink vs Medium Magenta

How close are they?

ΔE 33.9, loose: different enough that you should pick on purpose. Medium Magenta is noticeably darker, much more saturated than Fey Pink.

Can I use Medium Magenta instead of Fey Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Medium Magenta is noticeably darker, much more saturated than Fey Pink.

Can I use Fey Pink instead of Medium Magenta?

The same difference applies the other way round: Fey Pink is noticeably lighter, 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.