Gravestone Blue vs USAF Medium Gray

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. USAF Medium Gray is a little darker, more muted than Gravestone Blue.

Can I use USAF Medium Gray instead of Gravestone Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). USAF Medium Gray is a little darker, more muted than Gravestone Blue.

Can I use Gravestone Blue instead of USAF Medium Gray?

The same difference applies the other way round: Gravestone 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.