Zephyr Pink vs Ruby Red

How close are they?

ΔE 20.3, loose: different enough that you should pick on purpose. Ruby Red is noticeably darker, warmer than Zephyr Pink.

Can I use Ruby Red instead of Zephyr Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ruby Red is noticeably darker, warmer than Zephyr Pink.

Can I use Zephyr Pink instead of Ruby Red?

The same difference applies the other way round: Zephyr Pink is noticeably lighter, cooler.

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.