Cokum Copper vs Middle Stone

How close are they?

ΔE 19.5, loose: different enough that you should pick on purpose. Middle Stone is a little darker, much more muted, cooler than Cokum Copper.

Can I use Middle Stone instead of Cokum Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Middle Stone is a little darker, much more muted, cooler than Cokum Copper.

Can I use Cokum Copper instead of Middle Stone?

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