It has been suggested that the major advantage of trichromatic over dichromatic colour vision in primates is enhanced detection of red/yellow food items such as fruit against the dappled foliage of the forest. This hypothesis was tested by comparing the foraging ability of dichromatic and trichromatic Geoffroy's marmosets (Callithrix geoffroyi) for orange– and green–coloured cereal balls (Kix®) in a naturalized captive setting. Trichromatic marmosets found a significantly greater number of orange, but not green, Kix® than dichromatic marmosets when the food items were scattered on the floor of the cage (at a potential detection distance of up to 6 m from the animals). Under these conditions, trichromats but not dichromats found significantly more orange than green Kix®, an effect that was also evident when separately examining the data from the end of the trials, when the least conspicuous Kix® were left. In contrast, no significant differences among trichromats and dichromats were seen when the Kix® were placed in trays among green wood shavings (detection distance < 0.5 m). These results support an advantage for trichromats in detecting orange–coloured food items against foliage, and also suggest that this advantage may be less important at shorter distances. If such a foraging advantage for trichromats is present in the wild it might be sufficient to maintain the colour vision polymorphism seen in the majority of New World monkeys.