Onyx Skin vs Ashen Brown

How close are they?

ΔE 6.4, close: similar, with a visible difference side by side. Ashen Brown is a little lighter, cooler than Onyx Skin.

Can I use Ashen Brown instead of Onyx Skin?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Ashen Brown is a little lighter, cooler than Onyx Skin.

Can I use Onyx Skin instead of Ashen Brown?

The same difference applies the other way round: Onyx Skin is a little 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.