Primary Cyan vs Ultramarine Blue

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Ultramarine Blue is a little lighter, more muted than Primary Cyan.

Can I use Ultramarine Blue instead of Primary Cyan?

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

Can I use Primary Cyan instead of Ultramarine Blue?

The same difference applies the other way round: Primary Cyan 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.