In 2006, the NFL completely redesigned the shirts, going to a sleeker-looking uniform which, however, no longer identified a given official's position from the front. Also new for 2006 were black pants with a white stripe down the side to be worn in cold weather. These looser-fitting pants allow for layering of warmer clothes underneath. Beginning with the 2010 season, college officials and, in many states, high school officials, have the option of wearing the black cold-weather pants.
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Found the answer at: http://blogs.nfl.com/2009/03/26/hochuli-will-look-great-in-red/
2009 marks the 50th anniversary of the AFL. In games where the original eight teams meet, "Legacy Games", players and referees will wear the original AFL uniforms.
The NFL is celebrating the 50th anniversary of the American football league, and they started out wearing orange and white
for one to collect the excess sweat coming out of them and two it symbolises them as a referee... it is part of their uniform.
In the days of black and white tv, one team would wear a dark uniform and the other team would wear mostly white.