Hot Pink vs Primary Magenta

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Primary Magenta is noticeably darker, warmer than Hot Pink.

Can I use Primary Magenta instead of Hot Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Primary Magenta is noticeably darker, warmer than Hot Pink.

Can I use Hot Pink instead of Primary Magenta?

The same difference applies the other way round: Hot 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.