Duplication of an ancestral opsin gene occurred in the primate lineage (Old World primates) and subsequent mutations in the new copy resulted in two types of opsin, instead of just one.
Opsins are a group of light-sensitive proteins found in photoreceptor cells (cone cells) of the retina and they are the primary photopigments in primate eyes. Opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade.
The difference in colour vision between New and Old World primates results from differing arrangements of the photopigment genes on the X chromosome. In Old World primates the three photopigments are required for trichromatic colour vision and they are encoded by two or more X-chromosome photopigment genes and an autosomal photopigment gene. New World monkeys typically have only one X-chromosome pigment gene.