ΔE 15.8, loose: different enough that you should pick on purpose. Ancient Copper is a little darker, warmer than Iridescent Bright Gold (Fine). They are different kinds of paint: Golden Iridescent Bright Gold (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.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ancient Copper is a little darker, warmer than Iridescent Bright Gold (Fine). They are different kinds of paint.
The same difference applies the other way round: Iridescent Bright Gold (Fine) is a little lighter, cooler.
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 Bright Gold (Fine) on the colour wheel See Ancient Copper on the colour wheel
All matches: Iridescent Bright Gold (Fine) equivalents · Ancient Copper equivalents