Old Copper vs Ruby Red

How close are they?

ΔE 36.4, loose: different enough that you should pick on purpose. Ruby Red is a little darker, more saturated, cooler than Old Copper.

Can I use Ruby Red instead of Old Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Ruby Red is a little darker, more saturated, cooler than Old Copper.

Can I use Old Copper instead of Ruby Red?

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