Copper vs Ruby Red

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Ruby Red is a little lighter, more saturated, cooler than Copper.

Can I use Ruby Red instead of Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ruby Red is a little lighter, more saturated, cooler than Copper.

Can I use Copper instead of Ruby Red?

The same difference applies the other way round: Copper is a little darker, 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.