Red Copper vs Ancient Bronze

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Ancient Bronze is a little lighter, more muted, warmer than Red Copper.

Can I use Ancient Bronze instead of Red Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ancient Bronze is a little lighter, more muted, warmer than Red Copper.

Can I use Red Copper instead of Ancient Bronze?

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