ΔE 11.9, fair: the same colour family, but not a straight swap. Ancient Copper is a little darker, more saturated than Iridescent Copper (Fine). They are different kinds of paint: Golden Iridescent Copper (Fine) is a pearl paint and Vallejo Ancient Copper 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). Ancient Copper is a little darker, more saturated than Iridescent Copper (Fine). They are different kinds of paint.
The same difference applies the other way round: Iridescent Copper (Fine) is a little lighter, more muted.
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 Iridescent Copper (Fine) on the colour wheel See Ancient Copper on the colour wheel
All matches: Iridescent Copper (Fine) equivalents · Ancient Copper equivalents