ΔE 11.1, fair: the same colour family, but not a straight swap. Arctic Blue (Metal.) is noticeably lighter than Mithril.
Can I use Arctic Blue (Metal.) instead of Mithril?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Arctic Blue (Metal.) is noticeably lighter than Mithril.
Can I use Mithril instead of Arctic Blue (Metal.)?
The same difference applies the other way round: Mithril is noticeably darker.
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.