Dark Copper vs Amber Green

How close are they?

ΔE 33.0, loose: different enough that you should pick on purpose. Amber Green is a little darker, much more saturated, cooler than Dark Copper.

Can I use Amber Green instead of Dark Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Amber Green is a little darker, much more saturated, cooler than Dark Copper.

Can I use Dark Copper instead of Amber Green?

The same difference applies the other way round: Dark Copper is a little lighter, much 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.