ΔE 8.1, close: similar, with a visible difference side by side. Rusty Metal is a little darker, more saturated, cooler than Dark Copper.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Rusty Metal is a little darker, more saturated, cooler than Dark Copper.
The same difference applies the other way round: Dark Copper is a little lighter, more muted, warmer.
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 Dark Copper on the colour wheel See Rusty Metal on the colour wheel
All matches: Dark Copper equivalents · Rusty Metal equivalents