Cokum Copper vs Dark Sand

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Dark Sand is noticeably lighter, more muted, cooler than Cokum Copper.

Can I use Dark Sand instead of Cokum Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Dark Sand is noticeably lighter, more muted, cooler than Cokum Copper.

Can I use Cokum Copper instead of Dark Sand?

The same difference applies the other way round: Cokum Copper is noticeably darker, more saturated, 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.