Idrian Flesh vs Sanguine Base

How close are they?

ΔE 24.5, loose: different enough that you should pick on purpose. Sanguine Base is noticeably darker, cooler than Idrian Flesh.

Can I use Sanguine Base instead of Idrian Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Sanguine Base is noticeably darker, cooler than Idrian Flesh.

Can I use Idrian Flesh instead of Sanguine Base?

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