ΔE 7.8, close: similar, with a visible difference side by side. Saddle Brown is a little lighter, cooler than Onyx Skin.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Saddle Brown is a little lighter, cooler than Onyx Skin.
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.
See Onyx Skin on the colour wheel See Saddle Brown on the colour wheel
All matches: Onyx Skin equivalents · Saddle Brown equivalents