ΔE 11.3, fair: the same colour family, but not a straight swap. Iridescent Purple/Green is more saturated than Mithril. They are different kinds of paint: Army Painter Mithril is a metallic and Tamiya Iridescent Purple/Green 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 11). Iridescent Purple/Green is more saturated than Mithril. They are different kinds of paint.
The same difference applies the other way round: Mithril is 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 Iridescent Purple/Green on the colour wheel
All matches: Mithril equivalents · Iridescent Purple/Green equivalents