ΔE 27.3, loose: different enough that you should pick on purpose. Pearl Green is a little darker, much more saturated than Mithril. They are different kinds of paint: Army Painter Mithril is a metallic and Tamiya Pearl Green 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 27). Pearl Green is a little darker, much more saturated than Mithril. They are different kinds of paint.
The same difference applies the other way round: Mithril is a little lighter, much 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 Pearl Green on the colour wheel
All matches: Mithril equivalents · Pearl Green equivalents