Trichromatic colour vision, characterized by three retinal photopigments tuned to peak wavelengths of similar to 430 nm, similar to 535 nm and similar to 562 nm (refs 1, 2), has evolved convergently in catarrhine primates and one genus of New World monkey, the howlers (genus Alouatta)(3). This uniform capacity to discriminate red- green colours, which is not found in other mammals, has been proposed as advantageous for the long-range detection of either ripe fruits(4,5) or young leaves(6) (which frequently flush red in the tropics(7)) against a background of mature foliage(8,9). Here we show that four trichromatic primate species in Kibale Forest, Uganda, eat leaves that are colour discriminated only by red-greenness, a colour axis correlated with high protein levels and low toughness. Despite their divergent digestive systems, these primates have no significant interspecific differences in leaf colour selection. In contrast, eaten fruits were generally discriminated from mature leaves on both red-green and yellow-blue channels and also by their luminance, with a significant difference between chimpanzees and monkeys in fruit colour choice. Our results implicate leaf consumption, a critical food resource when fruit is scarce(10), as having unique value in maintaining trichromacy in catarrhines.