Pale Yellow vs Fairy Flesh

How close are they?

ΔE 20.4, loose: different enough that you should pick on purpose. Fairy Flesh is a little darker, much more saturated, warmer than Pale Yellow.

Can I use Fairy Flesh instead of Pale Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Fairy Flesh is a little darker, much more saturated, warmer than Pale Yellow.

Can I use Pale Yellow instead of Fairy Flesh?

The same difference applies the other way round: Pale Yellow is a little lighter, much more muted, 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.