Fulgurite Copper vs Scarab Red

How close are they?

ΔE 24.3, loose: different enough that you should pick on purpose. Scarab Red is a little darker, cooler than Fulgurite Copper.

Can I use Scarab Red instead of Fulgurite Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Scarab Red is a little darker, cooler than Fulgurite Copper.

Can I use Fulgurite Copper instead of Scarab Red?

The same difference applies the other way round: Fulgurite Copper is a little lighter, 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.