Red Copper vs Iridescent Copper (Coarse)

How close are they?

ΔE 22.0, loose: different enough that you should pick on purpose. Iridescent Copper (Coarse) is noticeably lighter, much more saturated, warmer than Red Copper. They are different kinds of paint: Army Painter Red Copper is a metallic and Golden Iridescent Copper (Coarse) is a pearl paint, so they won’t behave the same on the model even where the colour is close.

Can I use Iridescent Copper (Coarse) instead of Red Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Iridescent Copper (Coarse) is noticeably lighter, much more saturated, warmer than Red Copper. They are different kinds of paint.

Can I use Red Copper instead of Iridescent Copper (Coarse)?

The same difference applies the other way round: Red Copper is noticeably darker, much more muted, cooler.

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.