Yellow Oxide vs Fairy Flesh

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Fairy Flesh is noticeably lighter, more muted than Yellow Oxide.

Can I use Fairy Flesh instead of Yellow Oxide?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Fairy Flesh is noticeably lighter, more muted than Yellow Oxide.

Can I use Yellow Oxide instead of Fairy Flesh?

The same difference applies the other way round: Yellow Oxide is noticeably darker, 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.