Castes are the color coded groups the people of Mars are divided into. There are 14 castes, which each caste signified by their color, and each color signifies their ranking in the system, as well as their "job" in the society.
When the first colonists ventured forth from Earth to make their home on the moon, they created a hierarchy for labor. In time, they improved this hierarchy through genetic and surgical manipulation. The result was a color coded Society of perfect efficiency, dominated by a superior breed of humanity, the Golds.
It was they who rebelled against Earth, conquered her, and sowed the seeds of their Society across the solar system.