Related Rule
Germany
Practice Relating to Rule 62. Improper Use of Flags or Military Emblems, Insignia or Uniforms of the Adversary
Germany’s Military Manual (1992) provides: “It is prohibited to make improper use of … enemy … national flags, military insignia and uniforms.” 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, August 1992, § 473.
The manual further states:
Ruses of war are permissible also in naval warfare. Unlike land and aerial warfare, naval warfare permits the use of false flags or military emblems … Before opening fire, however, the true flag shall always be displayed. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, August 1992, § 1018.
Germany’s Law Introducing the International Crimes Code (2002) punishes anyone who, in connection with an international or non-international armed conflict, “makes improper use … of the flag or of the military insignia or of the uniform of the enemy … thereby causing a person’s death or serious injury”. 
Germany, Law Introducing the International Crimes Code, 2002, Article 1, § 10(2).
The Report on the Practice of Germany provides:
An official document of 1978 states that the improper use of uniforms can be seen as an act of perfidy. It continues by stating that there would be no breach in the case of wearing a uniform which is incomplete. International law contains no rules on the composition of uniforms. The important element is a certain designation or identification in order to comply with the principle of distinction. This designation does not necessarily have to be a uniform in the traditional sense. 
Report on the Practice of Germany, 1997, Chapter 2.6. (No source or document is cited.)