Old Red vs Pink Horror

How close are they?

ΔE 24.2, loose: different enough that you should pick on purpose. Pink Horror is noticeably darker, much more saturated, cooler than Old Red.

Can I use Pink Horror instead of Old Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Pink Horror is noticeably darker, much more saturated, cooler than Old Red.

Can I use Old Red instead of Pink Horror?

The same difference applies the other way round: Old Red is noticeably lighter, much more muted, 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.