The Color system of the Society is a form of social hierarchy devised and implemented by the early Golds during their conquering of the Solar System. It divides Humanity into fourteen distinct "Colors." By genetic engineering, the founders of The Society caused members of each Color to have skin and eyes to match their caste. Every person's vocation is dictated entirely by their Color. Reds are manual laborers, Pinks are social functionaries and pleasure slaves, Yellows are medical professionals, Violets are artisans and Obsidians are soldiers.
In addition to the genetic engineering that gives individuals the pigment to match their castes, a certain amount of social engineering exists in an attempt to prevent the desire for social mobility. Obsidians (for example) believe Golds to be literal gods, helping to ensure their loyalty to their masters.
While the genetic engineering is fairly perfect, the social engineering is proven time and again to be woefully inadequate. Throughout the novels, there are very few examples of specific low-color individuals who remain loyal to Gold, given a reasonable opportunity to rise against them. Even a surprising number of Golds seem to very-easily abandon the system that has allowed them to live as gods.
From left to right, top to bottom, the colors are: