In practice, it has been authorized to make use of national flags, insignia, and uniforms as a ruse. The foregoing rule (HR, art. 23, par. (f)) does not prohibit such employment, but does prohibit their improper use. It is certainly forbidden to employ them during combat, but their use at other times is not forbidden.  [emphasis in original]

Members of the armed forces of a party to the conflict and members of militias or volunteer corps forming part of such armed forces lose their right to be treated as prisoners of war whenever they deliberately conceal their status in order to pass behind the military lines of the enemy for the purpose of gathering military information or for the purpose of waging war by destruction of life or property. Putting on civilian clothes or the uniform of the enemy are examples of concealment of the status of a member of the armed forces. 

Members of the armed forces of a party to the conflict and members of militias or volunteer corps forming part of such armed forces lose their right to be treated as prisoners of war whenever they deliberately conceal their status in order to pass behind the military lines of the enemy for the purpose of gathering military information or for the purpose of waging war by destruction of life or property. Putting on civilian clothes or the uniform of the enemy are examples of concealment of the status of a member of the armed forces. Ground forces engaged in actual combat, in contrast to ground forces preparing for combat, are required to wear their own uniform or distinctive national insignia.

…

While combatant airmen are not absolutely required to wear a uniform or distinctive national insignia while flying in combat, improper use of the military insignia or uniform of the enemy is forbidden. Consequently, airmen should not wear the uniform or national insignia of the enemy while engaging in combat operations. Military aircraft, as entities of combat in aerial warfare, are also required to be marked with appropriate signs of their nationality and military character. 

At Sea. Naval surface and subsurface forces may fly enemy colors and display enemy markings to deceive the enemy. Warships must, however, display their true colors prior to an actual armed engagement.

In the Air. The use in combat of enemy markings by belligerent military aircraft is forbidden.

On Land. The law of land warfare does not prohibit the use by belligerent land forces of enemy flags, insignia, or uniforms to deceive the enemy either before or following an armed engagement. Combatants risk severe punishment, however, if they are captured while displaying enemy colors or insignia or wearing enemy uniforms in combat.

Similarly, combatants caught behind enemy lines wearing the uniform of their adversaries are not enentitled to prisoner-of-war status or protection and, historically, have been subjected to severe punishment. It is permissible, however, for downed aircrews and escaping prisoners of war to use enemy uniforms to evade capture, so long as they do not attack enemy forces, collect military intelligence, or engage in similar military operations while so attired. As a general rule, enemy markings should be removed from captured enemy equipment before it is used in combat. 

Naval surface and subsurface forces may fly enemy colors and display enemy markings to deceive the enemy. Warships must, however, display their true colors prior to an actual armed engagement.

The use in combat of enemy markings by belligerent military aircraft is forbidden.

The law of land warfare does not prohibit the use by belligerent land forces of enemy flags, insignia, or uniforms to deceive the enemy either before or following an armed engagement. Once an armed engagement begins, a belligerent is prohibited from deceiving an enemy by wearing an enemy uniform, or using enemy flags and insignia; combatants risk severe punishment if they are captured while displaying enemy colors or insignia or wearing enemy uniforms in combat.

Similarly, combatants caught behind enemy lines wearing the uniform of their adversaries run the risk of being denied prisoner-of-war status or protection and, historically, have been subjected to severe punishment. It is permissible, however, for downed aircrews and escaping prisoners of war to use enemy uniforms to evade capture, so long as they do not attack enemy forces, collect military intelligence, or engage in similar military operations while so attired.

Captured enemy equipment and supplies may be seized and used. Enemy markings, however, should be removed from captured enemy equipment before it is used in combat.