Idrian Flesh vs Nocturnal Red

How close are they?

ΔE 26.2, loose: different enough that you should pick on purpose. Nocturnal Red is noticeably darker, more muted, cooler than Idrian Flesh.

Can I use Nocturnal Red instead of Idrian Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Nocturnal Red is noticeably darker, more muted, cooler than Idrian Flesh.

Can I use Idrian Flesh instead of Nocturnal Red?

The same difference applies the other way round: Idrian Flesh is noticeably lighter, more saturated, 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.