This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Chris Murphy <murphyc@bionette.CGRB.ORST.EDU>
Chauner and Halstead (1990) in "The Tour de France Complete Book of Cycling"
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.