Fulgurite Copper vs Boiler Black

How close are they?

ΔE 47.8, loose: different enough that you should pick on purpose. Boiler Black is noticeably darker, much more muted than Fulgurite Copper.

Can I use Boiler Black instead of Fulgurite Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 48). Boiler Black is noticeably darker, much more muted than Fulgurite Copper.

Can I use Fulgurite Copper instead of Boiler Black?

The same difference applies the other way round: Fulgurite 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.