Leadbelcher vs Dark Copper

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Dark Copper is noticeably lighter, much more saturated than Leadbelcher.

Can I use Dark Copper instead of Leadbelcher?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Copper is noticeably lighter, much more saturated than Leadbelcher.

Can I use Leadbelcher instead of Dark Copper?

The same difference applies the other way round: Leadbelcher 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.