Rough Iron vs Metallic Black

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Metallic Black is a little lighter, more muted than Rough Iron.

Can I use Metallic Black instead of Rough Iron?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Metallic Black is a little lighter, more muted than Rough Iron.

Can I use Rough Iron instead of Metallic Black?

The same difference applies the other way round: Rough Iron is a little darker, more saturated.

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.