Light Blue vs Ulthuan Grey

How close are they?

ΔE 9.0, close: similar, with a visible difference side by side. Ulthuan Grey is a little darker, more muted than Light Blue.

Can I use Ulthuan Grey instead of Light Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Ulthuan Grey is a little darker, more muted than Light Blue.

Can I use Light Blue instead of Ulthuan Grey?

The same difference applies the other way round: Light Blue is a little lighter, 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.