ΔE 20.6, loose: different enough that you should pick on purpose. Idrian Flesh is much more saturated than Sylvaneth Bark.
Can I use Idrian Flesh instead of Sylvaneth Bark?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Idrian Flesh is much more saturated than Sylvaneth Bark.
Can I use Sylvaneth Bark instead of Idrian Flesh?
The same difference applies the other way round: Sylvaneth Bark is much more muted.
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.