ΔE 15.5, loose: different enough that you should pick on purpose. Fulgurite Copper is more muted than Golden Armour.
Can I use Fulgurite Copper instead of Golden Armour?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Fulgurite Copper is more muted than Golden Armour.
Can I use Golden Armour instead of Fulgurite Copper?
The same difference applies the other way round: Golden Armour is 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.