The Rainbow jersey is a special jersey worn by world champion cyclists during races.
It is mainly white with five horizontal bands in the UCI colours around the chest. From the bottom up the colours are: green, yellow, black, red and blue, the same colours that appear in the rings on the olympic flag. There are different jerseys for each discipline (road racing, track racing, cyclo-cross, BMX, and the types of mountain biking).
A cyclist must wear the jersey when competing in the same discipline, category and speciality that he is world champion for. For example, the world road race champion would wear the jersey while competing in stage races and one-day races, but would not during time trials. Similarly, on the track, the world individual pursuit champion would only wear the jersey when competing in other individual pursuit events.
A champion who does not wear the rainbow jersey can be fined 2500 to 5000 Swiss francs.
The team's sponsor pays a lot of money to have their name on each rider's jersey. They can still put their names on the world champion's jersey, but probably smaller because there is less space. Sponsors do not mind because the world champion gets a lot more publicity than ordinary riders.
A world champion who is leading a stage race wears the race-leader's jersey, not the world champion's, but only while he is the leader.