Copper vs Balthasar Gold

How close are they?

ΔE 19.6, loose: different enough that you should pick on purpose. Balthasar Gold is a little darker, more saturated, cooler than Copper.

Can I use Balthasar Gold instead of Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Balthasar Gold is a little darker, more saturated, cooler than Copper.

Can I use Copper instead of Balthasar Gold?

The same difference applies the other way round: 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.