ΔE 17.7, loose: different enough that you should pick on purpose. Royal Brown is much more muted, warmer than Khorne Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Royal Brown is much more muted, warmer than Khorne Red.
The same difference applies the other way round: Khorne Red is much 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.
See Khorne Red on the colour wheel See Royal Brown on the colour wheel
All matches: Khorne Red equivalents · Royal Brown equivalents