Ionic Blue vs Bismark's Grave

How close are they?

ΔE 28.6, loose: different enough that you should pick on purpose. Bismark's Grave is noticeably darker, much more muted than Ionic Blue.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Bismark's Grave is noticeably darker, much more muted than Ionic Blue.

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

The same difference applies the other way round: Ionic Blue is noticeably lighter, much 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.