Bismark's Grave vs Ahriman Blue

How close are they?

ΔE 8.4, close: similar, with a visible difference side by side. Ahriman Blue is a little lighter, more saturated, cooler than Bismark's Grave.

Can I use Ahriman Blue instead of Bismark's Grave?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Ahriman Blue is a little lighter, more saturated, cooler than Bismark's Grave.

Can I use Bismark's Grave instead of Ahriman Blue?

The same difference applies the other way round: Bismark's Grave is a little darker, 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.