Before treatment, monkeys were trained to perform a computerbased colour vision test, the Cambridge Colour Test4,5, which was modified for use with animals6 (Fig. 2a). Dichromats who are missing either the L- or the M-photopigment fail to distinguish from grey: colours near the so-called ‘spectral neutral point’ located in the bluegreen region of colour space (near dominant wavelength of 490 nm) and complementary colours near the ‘extra-spectral neutral point’ in the red-violet region (near dominant wavelength of 2499 nm). Whereas trichromats have the four main hue percepts blue, yellow, red and green, dichromats only have two percepts, nominally blue and yellow. Before treatment, two dichromatic monkeys complete