ΔE 12.8, fair: the same colour family, but not a straight swap. Iridescent Stainless Steel (Fine) is noticeably darker than Mithril. They are different kinds of paint: Army Painter Mithril is a metallic and Golden Iridescent Stainless Steel (Fine) is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Iridescent Stainless Steel (Fine) is noticeably darker than Mithril. They are different kinds of paint.
The same difference applies the other way round: Mithril 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 Mithril on the colour wheel See Iridescent Stainless Steel (Fine) on the colour wheel
All matches: Mithril equivalents · Iridescent Stainless Steel (Fine) equivalents