ΔE 51.5, loose: different enough that you should pick on purpose. Iron Hue is noticeably lighter, much more muted than Red Copper.
Can I use Iron Hue instead of Red Copper?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 51). Iron Hue is noticeably lighter, much more muted than Red Copper.
Can I use Red Copper instead of Iron Hue?
The same difference applies the other way round: Red 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.