ΔE 10.9, fair: the same colour family, but not a straight swap. Iridescent Copper (Fine) is a little lighter, more saturated than Red Copper. They are different kinds of paint: Army Painter Red Copper is a metallic and Golden Iridescent Copper (Fine) is a pearl paint, 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 11). Iridescent Copper (Fine) is a little lighter, more saturated than Red Copper. They are different kinds of paint.
The same difference applies the other way round: Red Copper is a little darker, 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 Red Copper on the colour wheel See Iridescent Copper (Fine) on the colour wheel
All matches: Red Copper equivalents · Iridescent Copper (Fine) equivalents