Dark Grey vs USAF Light Blue

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. USAF Light Blue is more saturated than Dark Grey.

Can I use USAF Light Blue instead of Dark Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). USAF Light Blue is more saturated than Dark Grey.

Can I use Dark Grey instead of USAF Light Blue?

The same difference applies the other way round: Dark Grey is more muted.

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.