Umberroot vs Idrian Flesh

How close are they?

ΔE 23.6, loose: different enough that you should pick on purpose. Idrian Flesh is noticeably lighter, much more saturated, warmer than Umberroot.

Can I use Idrian Flesh instead of Umberroot?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Idrian Flesh is noticeably lighter, much more saturated, warmer than Umberroot.

Can I use Umberroot instead of Idrian Flesh?

The same difference applies the other way round: Umberroot is noticeably darker, much more muted, cooler.

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.