Gemstone vs Metallic Red

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Metallic Red is a little lighter, cooler than Gemstone.

Can I use Metallic Red instead of Gemstone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Metallic Red is a little lighter, cooler than Gemstone.

Can I use Gemstone instead of Metallic Red?

The same difference applies the other way round: Gemstone is a little darker, 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.