Scarab Red vs Ruby Red

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Ruby Red is noticeably darker, cooler than Scarab Red.

Can I use Ruby Red instead of Scarab Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ruby Red is noticeably darker, cooler than Scarab Red.

Can I use Scarab Red instead of Ruby Red?

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