Gemstone Red vs Ruby Alchemy

How close are they?

ΔE 27.4, loose: different enough that you should pick on purpose. Ruby Alchemy is noticeably lighter, cooler than Gemstone Red.

Can I use Ruby Alchemy instead of Gemstone Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Ruby Alchemy is noticeably lighter, cooler than Gemstone Red.

Can I use Gemstone Red instead of Ruby Alchemy?

The same difference applies the other way round: Gemstone Red is noticeably 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.