Copper vs Gemstone Red

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Gemstone Red is noticeably darker, more saturated, cooler than Copper.

Can I use Gemstone Red instead of Copper?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Gemstone Red is noticeably darker, more saturated, cooler than Copper.

Can I use Copper instead of Gemstone Red?

The same difference applies the other way round: Copper is noticeably lighter, 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.