ΔE 21.0, loose: different enough that you should pick on purpose. Ancient Copper is noticeably lighter, more saturated, warmer than Iridescent Copper (Coarse). They are different kinds of paint: Golden Iridescent Copper (Coarse) 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 21). Ancient Copper is noticeably lighter, more saturated, warmer than Iridescent Copper (Coarse). They are different kinds of paint.
The same difference applies the other way round: Iridescent Copper (Coarse) is noticeably darker, more muted, 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 Copper (Coarse) on the colour wheel See Ancient Copper on the colour wheel
All matches: Iridescent Copper (Coarse) equivalents · Ancient Copper equivalents