Gemstone vs Nebula Copper

How close are they?

ΔE 42.0, loose: different enough that you should pick on purpose. Nebula Copper is noticeably lighter, much more saturated, warmer than Gemstone.

Can I use Nebula Copper instead of Gemstone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Nebula Copper is noticeably lighter, much more saturated, warmer than Gemstone.

Can I use Gemstone instead of Nebula Copper?

The same difference applies the other way round: Gemstone is noticeably darker, much more muted, cooler.

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.