Geneva Convention Protocol
The "Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts" signed in December 1977, includes some interesting flag-related material not only on use of the Red Cross and related flags but also other use of flags in wartime. In particular it provides for two additional flags of protection, one for civil defense installations and units and one for installations containing "dangerous forces."
Here's a summary:
Art 37 prohibits the killing, injuring or capturing of an adversary by resort to perfidy. Examples of perfidy mentioned in the Protocol are the feigning of an intent to negotiate under a flag of truce or of a surrender and the feigning of protected status by the use of signs, emblems or uniforms of the United Nations or of neutral or other States not Parties to the conflict.
Art 38 prohibits the improper use of the emblems, etc., provided for by the conventions and the Protocol or other "internationally recognized protective emblems, signs or signals, including the flag of truce" or to make any use of the distinctive emblem of the UN without the UN's authorization.
Art 39 prohibits the use in an armed conflict of the flags or other emblems of states not parties to the conflict, or of those of adverse parties while engaging in attacks or in order to "shield, favour, protect or impede military operations." However, this restriction explicitly does not affect the "existing generally recognized rules of international law applicable to . . . the conduct of armed conflict at sea," i.e., the use of false colors by naval vessels
Art 56 provides for a "special sign" for "works and installations containing dangerous forces." An annex to the Protocol defines the special sign as a group of three bright orange circles of equal size, placed on the same axis, the distance between each circle being one radius. When used on a flag, "the distance between the outer limits of the sign and the adjacent sides of the flag shall be one radius of a circle. The flag shall be rectangular and shall have a white ground." An image of it follows:
Note that "bright orange" is not further defined, nor are the proportions of the flag beyond the size and spacing of the circles.
Article 66 provides that the international distinctive sign of civil defence is an equilateral blue triangle on an orange ground when used for the protection of civil defence organizations, their personnel, buildings and matériel and for civilian shelters. An annex to the Protocol "recommends" that if the blue triangle is on a flag, the ground to the triangle be the orange flag, one of the angles of the triangle be pointed vertically upwards, and no angle of the triangle touch the edge of the orange ground. An image of it follows:
Note that no specific shades or proportions are prescribed by the Protocol.
The 1977 Protocols to the 1949 Geneva Convention provided in Article 66 that the international distinctive sign of civil defence is an equilateral blue triangle on an orange ground. I believe many civil defense organizations used a triangle emblem well before that, however. US civil defense facilities since the World War II period have been marked by a blue sign with a white triangle and the letters "CD" on the center forming a circle. The US Federal Emergency Management Agency seal is a variant of the US coat of arms with a white triangle on a blue roundel above the eagle's head.
This symbol is not used as a flag in Sweden. It is used as a sign put on walls of buildings, where you can find an air-raid shelter. The signs are maybe 10 cm x 10 cm, the orange and blue are often surrounded by white, and on the white some text, like "skyddsrum" (= air-raid shelter). When I was in school, I was taught that the symbol of an equilateral blue triangle on an orange ground was the international sign for an air-raid shelter.