Pure Copper vs Ruby Red

How close are they?

ΔE 26.0, loose: different enough that you should pick on purpose. Ruby Red is noticeably darker, much more saturated, cooler than Pure Copper.

Can I use Ruby Red instead of Pure Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Ruby Red is noticeably darker, much more saturated, cooler than Pure Copper.

Can I use Pure Copper instead of Ruby Red?

The same difference applies the other way round: Pure Copper is noticeably lighter, much more muted, 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.