Yellow Ochre vs Fairy Flesh

How close are they?

ΔE 21.0, loose: different enough that you should pick on purpose. Fairy Flesh is noticeably lighter, much more muted than Yellow Ochre.

Can I use Fairy Flesh instead of Yellow Ochre?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Fairy Flesh is noticeably lighter, much more muted than Yellow Ochre.

Can I use Yellow Ochre instead of Fairy Flesh?

The same difference applies the other way round: Yellow Ochre is noticeably darker, much more saturated.

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.