Infernal Red vs Khorne Red

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Khorne Red is noticeably darker, more muted than Infernal Red.

Can I use Khorne Red instead of Infernal Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Khorne Red is noticeably darker, more muted than Infernal Red.

Can I use Infernal Red instead of Khorne Red?

The same difference applies the other way round: Infernal Red is noticeably lighter, more saturated.

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.