Leadbelcher vs Bronzegrun

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. Bronzegrun is noticeably darker, much more saturated than Leadbelcher.

Can I use Bronzegrun instead of Leadbelcher?

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

Can I use Leadbelcher instead of Bronzegrun?

The same difference applies the other way round: Leadbelcher is noticeably lighter, 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.