Why aren't the undersides of military aircraft painted bluey-grey to camouflage them against the sky?
Shades of Blue-grey you say? So.......
The F15

(source: boeing.com)
The Mig21

The AWACS

(source: airforce-technology.com)
The Eurofighter

(source: ausairpower.net)
To name a few I could think of from the top of my head. Almost every military aircraft has a variant that has been painted along the blue-grey spectrum at some point or another.