Pink Horror vs Rust Brown

How close are they?

ΔE 28.3, loose: different enough that you should pick on purpose. Rust Brown is a little darker, more muted, warmer than Pink Horror.

Can I use Rust Brown instead of Pink Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Rust Brown is a little darker, more muted, warmer than Pink Horror.

Can I use Pink Horror instead of Rust Brown?

The same difference applies the other way round: Pink Horror is a little lighter, more saturated, 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.