True Copper vs Boiler Black

How close are they?

ΔE 44.2, loose: different enough that you should pick on purpose. Boiler Black is noticeably darker, much more muted than True Copper.

Can I use Boiler Black instead of True Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 44). Boiler Black is noticeably darker, much more muted than True Copper.

Can I use True Copper instead of Boiler Black?

The same difference applies the other way round: True Copper is noticeably lighter, much 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.