ΔE 15.3, loose: different enough that you should pick on purpose. Copper (Metallic) is noticeably lighter than Mythril Blade.
Can I use Copper (Metallic) instead of Mythril Blade?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Copper (Metallic) is noticeably lighter than Mythril Blade.
Can I use Mythril Blade instead of Copper (Metallic)?
The same difference applies the other way round: Mythril Blade 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.