Spectrum Blue vs Ulthuan Grey

How close are they?

ΔE 8.0, close: similar, with a visible difference side by side. Ulthuan Grey is a little lighter than Spectrum Blue.

Can I use Ulthuan Grey instead of Spectrum Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Ulthuan Grey is a little lighter than Spectrum Blue.

Can I use Spectrum Blue instead of Ulthuan Grey?

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

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.