Castellax Bronze vs Copper

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Copper is a little lighter, more saturated than Castellax Bronze.

Can I use Copper instead of Castellax Bronze?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Copper is a little lighter, more saturated than Castellax Bronze.

Can I use Castellax Bronze instead of Copper?

The same difference applies the other way round: Castellax Bronze is a little darker, more muted.

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.