The blue UN beret/helmet, cap is worn by members of the United Nations peacekeeping force.
In 1947, General Assembly resolution 167 (II) approved light blue for the United Nations flag. This distinctive color came to represent the UN.
But the question arose about how could people differentiate peacekeepers from the soldiers who were actually fighting one another? Military uniforms could look a lot alike but it was decided to have the UN peacekeepers wear a bright blue helmet or hat to make it easy to quickly recognize them.
History: The origins for the blue beret/helmet go back to the first UN armed peace support operation in the Sinai Peninsula in 1956 (Israeli forces launched an attack on Egypt and occupied Sinai and the Gaza Strip), when the United Nations emergency force [UNEF] was established. Up to then, there was no common dress or insignia.
Canada provided a sizable contingent to UNEF and they were uniformed the same as the British forces who were one of the belligerents in the conflict. To distinguish themselves as noncombatants, the Canadians painted their helmet liners light sky-blue and the vehicles white, the same colors as the United Nations flag. The peaked caps, which were standard Canadian Army issue, were dyed blue and issued as a common head covering provided to peacekeepers. The blue beret came to be the most recognized insignia of UN troops.
These photos represent items in the CSS Museum: UN Beret (top), helmet cover (middle), baseball style cap (bottom).