Why do golf flags differ in color?

A topic discussed in this week's leadership class in FairWays to Leadership was empowering yourself and your playing partners with the information you need to make good decisions on the golf course. One of those pieces of information was knowing the hole positions on the green based on the color of the flag flying on the pin. Common flag colors include red, white, and blue and these colors typically refer to the hole being in the front, middle or back of the green, respectively. Has anyone seen other flag colors, and if so, what did they represent?