Dark Red vs Khorne Red

How close are they?

ΔE 9.6, close: similar, with a visible difference side by side. Khorne Red is a little darker, more muted, cooler than Dark Red.

Can I use Khorne Red instead of Dark Red?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Khorne Red is a little darker, more muted, cooler than Dark Red.

Can I use Dark Red instead of Khorne Red?

The same difference applies the other way round: Dark Red is a little lighter, more saturated, 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.