Chauner and Halstead (1990) in "The Tour de France Complete Book of Cycling"
explain:

YELLOW Jersey -- Overall leader, first awarded during the 1919 race (TdF
       started in 1903); yellow to match the paper used to print L'Auto
       (Automobile Cyclisme), a French newspaper about bike racing.

POLKADOT Jersey (White w/red dots) -- Best climber, determined by points
         scored by the first 3 to 15 riders finishing selected mountain
         stages (number of riders awarded points varies with the
         difficulty of the stage). First awarded 1933.

GREEN Jersey -- Points jersey, usually won by sprinter-types, with points
      given to the first 25 riders to finish each stage. First awarded 1953.

YELLOW Hats -- First place team, determined by combined elapsed times of the
       the team's top 3 riders.

In the event of a rider leading the race and also deserving one of the other
jerseys, the race leader wears yellow, and the 2nd place in the category wears
the category jersey.

Some parts © 2009 Advameg, Inc.