ΔE 31.1, loose: different enough that you should pick on purpose. Primary Magenta is noticeably lighter, much more saturated, cooler than Khorne Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Primary Magenta is noticeably lighter, much more saturated, cooler than Khorne Red.
The same difference applies the other way round: Khorne Red is noticeably darker, 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.
See Khorne Red on the colour wheel See Primary Magenta on the colour wheel
All matches: Khorne Red equivalents · Primary Magenta equivalents