ΔE 11.6, fair: the same colour family, but not a straight swap. Copper (Metallic) is a little darker, more muted than Interference Blue (Fine). They are different kinds of paint: Golden Interference Blue (Fine) is a pearl paint and Vallejo Copper (Metallic) is a metallic, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Copper (Metallic) is a little darker, more muted than Interference Blue (Fine). They are different kinds of paint.
The same difference applies the other way round: Interference Blue (Fine) is a little lighter, 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.
See Interference Blue (Fine) on the colour wheel See Copper (Metallic) on the colour wheel
All matches: Interference Blue (Fine) equivalents · Copper (Metallic) equivalents