Weathered Blue vs Grey Blue

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Grey Blue is a little darker, warmer than Weathered Blue.

Can I use Grey Blue instead of Weathered Blue?

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

Can I use Weathered Blue instead of Grey Blue?

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

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.