After 20 weeks, the monkeys' colour skills improved dramatically, indicating that Dalton and Sam had acquired the ability to see in three shades.
Both monkeys have retained this skill for more than two years with no apparent side effects, the researchers report in Nature.
Adding the missing gene was sufficient to restore full colour vision without further rewiring of the brain even though the monkeys had been colour blind since birth.
"There is this plasticity still in the brain and it is possible to treat cone defects with gene therapy," says Alexander Smith, a molecular biologist and vision researcher at University College London, who did not contribute to the study.
"It doesn't seem like new neural connections have to be formed," says Komáromy.
"You can add an additional cone opsin pigment and the neural circuitry and visual pathways can deal with it."
Three human gene therapy trials are currently under way for loss of sight due to serious degeneration of the retina.
These phase I safety studies injected a similar type of virus vector (but carrying a different gene) behind the retina as in the monkeys, and people treated have shown no serious adverse effects more than a year after, with some participants reporting marked improvements in vision.
These first human trials — which repair rods, a different type of photoreceptor cell — can be seen as a safety benchmark for any future treatment of cone diseases and colour blindness in humans, says Neitz.
"The biggest issue is that people who are colour blind have very good vision," Neitz says.
"So before people are going to want to treat colour blindness you're going to want to ensure that this is completely safe, and that's going to take some work."
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