Metallic Brown vs Ancient Copper

How close are they?

ΔE 9.6, close: similar, with a visible difference side by side. Ancient Copper is a little darker than Metallic Brown.

Can I use Ancient Copper instead of Metallic Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Ancient Copper is a little darker than Metallic Brown.

Can I use Metallic Brown instead of Ancient Copper?

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