Leadbelcher vs Old Brass

How close are they?

ΔE 37.9, loose: different enough that you should pick on purpose. Old Brass is noticeably darker, more saturated than Leadbelcher.

Can I use Old Brass instead of Leadbelcher?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Old Brass is noticeably darker, more saturated than Leadbelcher.

Can I use Leadbelcher instead of Old Brass?

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