Red–green colour blindness, which results from the absence of either the long- (L) or the middle- (M) wavelength-sensitive visual photopigments, is the most common single locus genetic disorder. Here we explore the possibility of curing colour blindness using gene therapy in experiments on adult monkeys that had been colour blind since birth. A third type of cone pigment was added to dichromatic retinas, providing the receptoral basis for trichro- matic colour vision. This opened a new avenue to explore the requirements for establishing the neural circuits for a new dimen- sion of colour sensation. Classic visual deprivation experiments1 have led to the expectation that neural connections established during development would not appropriately process an input that was not present from birth. Therefore, it was believed that the treatment of congenital vision disorders would be ineffective unless administered to the very young. However, here we show that the addition of a third opsin in adult red–green colour- deficient primates was sufficient to produce trichromatic colour vision behaviour. Thus, trichromacy can arise from a single addi- tion of a third cone class and it does not require an early develop- mental process. This provides a positive outlook for the potential of gene therapy to cure adult vision disorders.