Dark Copper vs Crimson Magenta

How close are they?

ΔE 41.1, loose: different enough that you should pick on purpose. Crimson Magenta is much more saturated, cooler than Dark Copper.

Can I use Crimson Magenta instead of Dark Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 41). Crimson Magenta is much more saturated, cooler than Dark Copper.

Can I use Dark Copper instead of Crimson Magenta?

The same difference applies the other way round: Dark Copper is 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.