Bright Gold vs Ancient Copper

How close are they?

ΔE 26.3, loose: different enough that you should pick on purpose. Ancient Copper is a little darker, warmer than Bright Gold.

Can I use Ancient Copper instead of Bright Gold?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Ancient Copper is a little darker, warmer than Bright Gold.

Can I use Bright Gold instead of Ancient Copper?

The same difference applies the other way round: Bright Gold is a little lighter, 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.