Ultramarines Blue vs Storm Blue

How close are they?

ΔE 9.2, close: similar, with a visible difference side by side. Storm Blue is more muted than Ultramarines Blue.

Can I use Storm Blue instead of Ultramarines Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Storm Blue is more muted than Ultramarines Blue.

Can I use Ultramarines Blue instead of Storm Blue?

The same difference applies the other way round: Ultramarines Blue is 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.