Neptunus Blue vs Ultramarine Blue

How close are they?

ΔE 8.0, close: similar, with a visible difference side by side. Ultramarine Blue is more muted than Neptunus Blue.

Can I use Ultramarine Blue instead of Neptunus Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Ultramarine Blue is more muted than Neptunus Blue.

Can I use Neptunus Blue instead of Ultramarine Blue?

The same difference applies the other way round: Neptunus 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.