Alpha Blue vs Faded Ultramarine

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Faded Ultramarine is a little lighter, more muted than Alpha Blue.

Can I use Faded Ultramarine instead of Alpha Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Faded Ultramarine is a little lighter, more muted than Alpha Blue.

Can I use Alpha Blue instead of Faded Ultramarine?

The same difference applies the other way round: Alpha 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.