Mournfang Brown vs Idrian Flesh

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Idrian Flesh is noticeably lighter, more saturated, warmer than Mournfang Brown.

Can I use Idrian Flesh instead of Mournfang Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Idrian Flesh is noticeably lighter, more saturated, warmer than Mournfang Brown.

Can I use Mournfang Brown instead of Idrian Flesh?

The same difference applies the other way round: Mournfang Brown is noticeably darker, 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.