Metallic Medium vs Obsidian Black

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Obsidian Black is a little darker than Metallic Medium.

Can I use Obsidian Black instead of Metallic Medium?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Obsidian Black is a little darker than Metallic Medium.

Can I use Metallic Medium instead of Obsidian Black?

The same difference applies the other way round: Metallic Medium 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.