First of all American's didn't fly any Gray airplanes during WWII. They were pretty much either Army Green, Navy Blue (Hence the pic), or Silver. The National Aircraft Insignia is place on all AMERICAN military aircraft as the Air Force did not exist during WWII, only the Army Air Corps as well as the Naval Air Force. Had you said you would put an Iron Cross on the car I wouldn't have said anything cause the Germans flew gray aircraft during WWII.
Not on Topic, but oh well.
Either way Silverstone and United Gray are very similar. To me I think United Gray is slightly darker.