Fulgurite Copper vs Copper Fire

How close are they?

ΔE 20.0, loose: different enough that you should pick on purpose. Copper Fire is noticeably lighter, cooler than Fulgurite Copper.

Can I use Copper Fire instead of Fulgurite Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Copper Fire is noticeably lighter, cooler than Fulgurite Copper.

Can I use Fulgurite Copper instead of Copper Fire?

The same difference applies the other way round: Fulgurite Copper is noticeably darker, warmer.

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.