Dwarven Iron vs Beetle Green

How close are they?

ΔE 37.2, loose: different enough that you should pick on purpose. Beetle Green is noticeably lighter, much more saturated than Dwarven Iron.

Can I use Beetle Green instead of Dwarven Iron?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Beetle Green is noticeably lighter, much more saturated than Dwarven Iron.

Can I use Dwarven Iron instead of Beetle Green?

The same difference applies the other way round: Dwarven Iron is noticeably 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.