Bismark's Grave vs Imrik Blue

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Imrik Blue is a little lighter, much more saturated, warmer than Bismark's Grave.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Imrik Blue is a little lighter, much more saturated, warmer than Bismark's Grave.

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

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