Copper vs Ruby Red

How close are they?

ΔE 27.5, loose: different enough that you should pick on purpose. Ruby Red is a little darker, more muted, cooler than Copper.

Can I use Ruby Red instead of Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Ruby Red is a little darker, more muted, cooler than Copper.

Can I use Copper instead of Ruby Red?

The same difference applies the other way round: Copper is a little lighter, more saturated, 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.