ΔE 22.1, loose: different enough that you should pick on purpose. Beetle Green is a little lighter, much more saturated than Mythril Blade.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Beetle Green is a little lighter, much more saturated than Mythril Blade.
The same difference applies the other way round: Mythril Blade is a little darker, 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 Mythril Blade on the colour wheel See Beetle Green on the colour wheel
All matches: Mythril Blade equivalents · Beetle Green equivalents