ΔE 15.1, loose: different enough that you should pick on purpose. Iridescent Stainless Steel (Coarse) is noticeably darker than Runefang Steel. They are different kinds of paint: Citadel Runefang Steel is a metallic and Golden Iridescent Stainless Steel (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 15). Iridescent Stainless Steel (Coarse) is noticeably darker than Runefang Steel. They are different kinds of paint.
The same difference applies the other way round: Runefang Steel is noticeably lighter.
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 Runefang Steel on the colour wheel See Iridescent Stainless Steel (Coarse) on the colour wheel
All matches: Runefang Steel equivalents · Iridescent Stainless Steel (Coarse) equivalents