Obsidian Skin vs Necro Grey

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Necro Grey is a little darker than Obsidian Skin.

Can I use Necro Grey instead of Obsidian Skin?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Necro Grey is a little darker than Obsidian Skin.

Can I use Obsidian Skin instead of Necro Grey?

The same difference applies the other way round: Obsidian Skin 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.