Desolate Brown vs Orc Skin

How close are they?

ΔE 19.7, loose: different enough that you should pick on purpose. Orc Skin is a little darker, cooler than Desolate Brown.

Can I use Orc Skin instead of Desolate Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Orc Skin is a little darker, cooler than Desolate Brown.

Can I use Desolate Brown instead of Orc Skin?

The same difference applies the other way round: Desolate Brown is a little 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.