Ultramarine Blue vs Imperial Blue

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Imperial Blue is a little lighter, more muted than Ultramarine Blue.

Can I use Imperial Blue instead of Ultramarine Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Imperial Blue is a little lighter, more muted than Ultramarine Blue.

Can I use Ultramarine Blue instead of Imperial Blue?

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