Hot Pink vs RLM 23 Red

How close are they?

ΔE 34.9, loose: different enough that you should pick on purpose. RLM 23 Red is noticeably darker, more saturated, warmer than Hot Pink.

Can I use RLM 23 Red instead of Hot Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 35). RLM 23 Red is noticeably darker, more saturated, warmer than Hot Pink.

Can I use Hot Pink instead of RLM 23 Red?

The same difference applies the other way round: Hot Pink is noticeably lighter, more muted, 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.