How are jokers colored in decks with more than two jokers?
There is no common convention for 4 Jokers.

According to Wikipedia 


  There are usually two Jokers per deck, often noticeably different. For instance, Bicycle Playing Cards prints their company's guarantee claim on only one. More common traits are the appearance of colored and black/non-colored Jokers.


and other websites, there are normally only two jokers. The qualities of which, including appearance were probably borrowed from the Tarrot trump cards the Magacian/Juggler and (which is red in color, and outranks) the Fool (which is black in color), both of which have the apperance of a court jester.