Pure Copper vs Ancient Copper

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. Ancient Copper is much more saturated, warmer than Pure Copper.

Can I use Ancient Copper instead of Pure Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ancient Copper is much more saturated, warmer than Pure Copper.

Can I use Pure Copper instead of Ancient Copper?

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