ΔE 19.6, loose: different enough that you should pick on purpose. Iridescent Copper Light (Fine) is a little darker, more saturated, warmer than Balthasar Gold. They are different kinds of paint: Citadel Balthasar Gold is a metallic and Golden Iridescent Copper Light (Fine) is a pearl paint, 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 20). Iridescent Copper Light (Fine) is a little darker, more saturated, warmer than Balthasar Gold. They are different kinds of paint.
The same difference applies the other way round: Balthasar Gold is a little lighter, 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 Balthasar Gold on the colour wheel See Iridescent Copper Light (Fine) on the colour wheel
All matches: Balthasar Gold equivalents · Iridescent Copper Light (Fine) equivalents