Fahrenheit vs Ruby Red

How close are they?

ΔE 22.8, loose: different enough that you should pick on purpose. Ruby Red is much more muted, cooler than Fahrenheit.

Can I use Ruby Red instead of Fahrenheit?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Ruby Red is much more muted, cooler than Fahrenheit.

Can I use Fahrenheit instead of Ruby Red?

The same difference applies the other way round: Fahrenheit is much more saturated, 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.