XV-88 vs Cokum Copper

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Cokum Copper is noticeably lighter, more saturated, cooler than XV-88.

Can I use Cokum Copper instead of XV-88?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Cokum Copper is noticeably lighter, more saturated, cooler than XV-88.

Can I use XV-88 instead of Cokum Copper?

The same difference applies the other way round: XV-88 is noticeably darker, 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.