Opsins provide an excellent model system for studying evolutionary interconnections at genetic, phenotypic and behavioural levels. Primates have evolved a unique ability for trichromatic colour vision from a dichromatic mammalian ancestor. This was accomplished via allelic differentiation (e.g. most New World monkeys) or gene duplication (e.g. Old World primates) of the middle to long-wavelength sensitive (M/LWS) opsin gene. However, questions remain regarding the behavioural adaptations of primate trichromacy. Allelic differentiation of the M/LWS opsins results in extensive colour vision variability in New World monkeys, where trichromats and dichromats are found in the same breeding population, enabling us to directly compare visual performances among different colour vision phenotypes. Thus, New World monkeys can serve as an excellent model to understand and evaluate the adaptive significance of primate trichromacy in a behavioural context. In this chapter, we summarise recent findings on colour vision evolution in vertebrates, with special emphasis on primates, and introduce our genetic and behavioural study on primate colour vision polymorphism and adaptation.