ΔE 18.0, loose: different enough that you should pick on purpose. Iridescent Silver (Fine) is noticeably lighter than Ironbreaker. They are different kinds of paint: Citadel Ironbreaker is a metallic and Golden Iridescent Silver (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 18). Iridescent Silver (Fine) is noticeably lighter than Ironbreaker. They are different kinds of paint.
The same difference applies the other way round: Ironbreaker 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 Ironbreaker on the colour wheel See Iridescent Silver (Fine) on the colour wheel
All matches: Ironbreaker equivalents · Iridescent Silver (Fine) equivalents