ΔE 23.8, loose: different enough that you should pick on purpose. Interference Blue (Fine) is noticeably lighter, more saturated than Mithril. They are different kinds of paint: Army Painter Mithril is a metallic and Golden Interference Blue (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 24). Interference Blue (Fine) is noticeably lighter, more saturated than Mithril. They are different kinds of paint.
The same difference applies the other way round: Mithril is noticeably darker, more muted.
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 Interference Blue (Fine) on the colour wheel
All matches: Mithril equivalents · Interference Blue (Fine) equivalents