Related Rule
United States of America
Practice Relating to Rule 63. Use of Flags or Military Emblems, Insignia or Uniforms of Neutral or Other States Not Party to the Conflict
The US Air Force Pamphlet (1976) specifies: “Military aircraft may not bear … markings of neutral aircraft while engaging in combat.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 7-4.
The US Naval Handbook (1995) states:
At Sea. Under the customary international law of naval warfare, it is permissible for a belligerent warship to fly false colors and disguise its outward appearance in other ways in order to deceive the enemy into believing the vessel is of neutral nationality or is other than a warship. However, it is unlawful for a warship to go into action without first showing her true colors. Use of neutral flags, insignia, or uniforms during an actual armed engagement at sea is, therefore, forbidden.
In the Air. Use in combat of false or deceptive markings to disguise belligerent military aircraft as being of neutral nationality is prohibited.
On Land. The law of armed conflict applicable to land warfare has no rule of law analogous to that which permits belligerent warships to display neutral colors. Belligerents engaged in armed conflict on land are not permitted to use the flags, insignia, or uniforms of a neutral nation to deceive the enemy. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 12.3.
The US Naval Handbook (2007) states:
12.3 Neutral Flags, Insignia and Uniforms
12.3.1 At Sea
Under the customary international law of naval warfare, it is permissible for a belligerent warship to fly false colors and disguise its outward appearance in other ways in order to deceive the enemy into believing the vessel is of neutral nationality or is other than a warship. However, it is unlawful for a warship to go into action without first showing her true colors. Use of neutral flags, insignia, or uniforms during an actual armed engagement at sea is forbidden.
12.3.2 In the Air
Use in combat of false or deceptive markings to disguise belligerent military aircraft as being of neutral nationality is prohibited.
12.3.3 On Land
The law of armed conflict applicable to land warfare has no rule of law analogous to that which permits belligerent warships to display neutral colors. Belligerents engaged in armed conflict on land are not permitted to use the flags, insignia, or uniforms of a neutral nation to deceive the enemy. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, §§ 12.3, 12.3.1, 12.3.2 and 12.3.3.