Fairy Pink vs Light Magenta

How close are they?

ΔE 29.6, loose: different enough that you should pick on purpose. Light Magenta is noticeably lighter, warmer than Fairy Pink.

Can I use Light Magenta instead of Fairy Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Light Magenta is noticeably lighter, warmer than Fairy Pink.

Can I use Fairy Pink instead of Light Magenta?

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