Ultramarine Blue vs Blue

How close are they?

ΔE 9.7, close: similar, with a visible difference side by side. Blue is a little lighter, cooler than Ultramarine Blue.

Can I use Blue instead of Ultramarine Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Blue is a little lighter, cooler than Ultramarine Blue.

Can I use Ultramarine Blue instead of Blue?

The same difference applies the other way round: Ultramarine Blue is a little darker, 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.