The reason as to why the United Nations troops during the Korean War did not wear the now classic blue helmets and white vehicles were due to the simple fact that it had yet to be "invented".

The aftermath of the Suez Crisis, which took place in 1956, brought forth the first proper peacekeeping mission in the history of the United Nations. The mission was to uphold the peace on the armistice demarcation lines on the Sinai peninsula and to monitor the withdrawal of Israeli troops from Egyptian territory.

This peacekeeping force, dubbed the United Nations Emergency Force (UNEF), consisted of contingents of 10 different countries of which the most relevant to answer your question is Canada. One of the belligerent countries involved in the Suez Crisis had been the United Kingdom. The uniform of the Canadian soldiers that were involved in this particular mission were visually almost identical to British uniforms. The problem here is quite clear: How do you prevent Egyptian soldiers and civilians from mistaking Canadian soldiers for British soldiers that they had just fought against?

This issue of identification as well as the need for a clearly identifiable way to present all troop contingents as peacekeepers led to the creation of the infamous blue helmet for the UNEF and set the pattern for how future peacekeeping troops under the flag of the United Nations would look like in terms of identifiable gear: the blue helmet, the white vehicles, the blue berets and the United Nation patches all had their origin in the UNEF.

I have a following up question: today South Korean and American troops guard the DMZ but aren't those supposed to be UN troops? Or did the UN police action end at a certain time?

Please can you help me with a book which gives the origins of the U.N as well as the politics which governs its day to day decisions. Who has a major say in these decisions and why ?