ΔE 6.8, close: similar, with a visible difference side by side. Obsidian Skin is a little darker than Slate Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Obsidian Skin is a little darker than Slate Grey.
The same difference applies the other way round: Slate Grey is a little lighter.
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 Slate Grey on the colour wheel See Obsidian Skin on the colour wheel
All matches: Slate Grey equivalents · Obsidian Skin equivalents