Khorne Red vs Idrian Flesh

How close are they?

ΔE 28.4, loose: different enough that you should pick on purpose. Idrian Flesh is noticeably lighter, warmer than Khorne Red.

Can I use Idrian Flesh instead of Khorne Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Idrian Flesh is noticeably lighter, warmer than Khorne Red.

Can I use Khorne Red instead of Idrian Flesh?

The same difference applies the other way round: Khorne Red is noticeably darker, 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.