Onyx Skin vs Chestnut Brown

How close are they?

ΔE 8.1, close: similar, with a visible difference side by side. Chestnut Brown is more saturated, cooler than Onyx Skin.

Can I use Chestnut Brown instead of Onyx Skin?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Chestnut Brown is more saturated, cooler than Onyx Skin.

Can I use Onyx Skin instead of Chestnut Brown?

The same difference applies the other way round: Onyx Skin is more muted, 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.