Dark Green Brown vs Copper Brown

How close are they?

ΔE 20.4, loose: different enough that you should pick on purpose. Copper Brown is a little darker, more muted than Dark Green Brown.

Can I use Copper Brown instead of Dark Green Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Copper Brown is a little darker, more muted than Dark Green Brown.

Can I use Dark Green Brown instead of Copper Brown?

The same difference applies the other way round: Dark Green Brown is a little lighter, more saturated.

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.