Dark Elder Bronze vs Copper

How close are they?

ΔE 27.4, loose: different enough that you should pick on purpose. Copper is noticeably lighter, much more saturated, cooler than Dark Elder Bronze.

Can I use Copper instead of Dark Elder Bronze?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Copper is noticeably lighter, much more saturated, cooler than Dark Elder Bronze.

Can I use Dark Elder Bronze instead of Copper?

The same difference applies the other way round: Dark Elder Bronze is noticeably darker, much more muted, warmer.

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.