Fairy Pink vs Plum

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Plum is a little darker, cooler than Fairy Pink.

Can I use Plum instead of Fairy Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Plum is a little darker, cooler than Fairy Pink.

Can I use Fairy Pink instead of Plum?

The same difference applies the other way round: Fairy Pink is a little lighter, 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.