Fulgurite Copper vs Iron Hue

How close are they?

ΔE 51.0, loose: different enough that you should pick on purpose. Iron Hue is noticeably lighter, much more muted than Fulgurite Copper.

Can I use Iron Hue instead of Fulgurite Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 51). Iron Hue is noticeably lighter, much more muted than Fulgurite Copper.

Can I use Fulgurite Copper instead of Iron Hue?

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