Here we present a theory on the origin of allopatric primate species that follows - at least in Neotropical primates - the irreversible trend to albinotic skin and coat color, called "metachromic bleaching." It explains why primates constitute such an exceptionally diverse, species-rich, and colorful Order in the Class Mammalia. The theory is in tune with the principle of evolutionary change in tegumentary colors called "metachromism," a hypothesis propounded by the late Philip Hershkovitz. Metachromism holds the evolutionary change in hair, skin, and eye melanins following an orderly and irreversible sequence that ends in loss of pigment becoming albinotic, cream to silvery or white. In about all extant sociable Neotropical monkeys we identified an irreversible trend according to which metachromic varieties depart from the saturated eumelanin (agouti, black or blackish brown) archetypic form and then speciate into allopatric taxa following the trend to albinotic skin and coat color. Speciation goes either along the eumelanin pathway (from gray to silvery to cream to white), or the pheomelanin pathway (from red to orange to yellow to white), or a combination of the two. The theory represents a new evolutionary concept that seems to act indefinitely in a non-adaptive way in the population dynamics of male-hierarchic societies of all sociable primates that act territorial. We have tested the theory in all 17 extant Neotropical monkey genera. Our theory suggests the trend to allopatry among metachromic varieties in a social group or population to be the principal behavioral factor that empowers metachromic processes in sociable Neotropical monkeys. It may well represent the principal mechanism behind speciation, radiation, niche separation, and phylogeography in all sociable primates that hold male-defended territories. We urge field biologists who study primate distributions, demography and phylogeography in the Old World to take our theory to the test in the equally colorful Catarrhini.