ΔE 16.8, loose: different enough that you should pick on purpose. Pearl Grey is noticeably lighter than Ironbreaker. They are different kinds of paint: Citadel Ironbreaker is a metallic and Scale75 Pearl Grey 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 17). Pearl Grey 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 Pearl Grey on the colour wheel
All matches: Ironbreaker equivalents · Pearl Grey equivalents