Idrian Flesh vs Tan

How close are they?

ΔE 19.2, loose: different enough that you should pick on purpose. Tan is noticeably lighter, cooler than Idrian Flesh.

Can I use Tan instead of Idrian Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Tan is noticeably lighter, cooler than Idrian Flesh.

Can I use Idrian Flesh instead of Tan?

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