ΔE 15.7, loose: different enough that you should pick on purpose. Iridescent Copper Light (Coarse) is noticeably lighter than Castellax Bronze. They are different kinds of paint: Citadel Castellax Bronze is a metallic and Golden Iridescent Copper Light (Coarse) 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 16). Iridescent Copper Light (Coarse) is noticeably lighter than Castellax Bronze. They are different kinds of paint.
The same difference applies the other way round: Castellax Bronze is noticeably darker.
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 Castellax Bronze on the colour wheel See Iridescent Copper Light (Coarse) on the colour wheel
All matches: Castellax Bronze equivalents · Iridescent Copper Light (Coarse) equivalents