ΔE 12.7, fair: the same colour family, but not a straight swap. Mythril Blade is noticeably lighter than Plate Mail Metal.
Can I use Mythril Blade instead of Plate Mail Metal?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Mythril Blade is noticeably lighter than Plate Mail Metal.
Can I use Plate Mail Metal instead of Mythril Blade?
The same difference applies the other way round: Plate Mail Metal 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.