Grey-Blue vs Frost Blue

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Frost Blue is a little lighter, cooler than Grey-Blue.

Can I use Frost Blue instead of Grey-Blue?

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

Can I use Grey-Blue instead of Frost Blue?

The same difference applies the other way round: Grey-Blue is a little darker, warmer.

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.