The ability of monkeys to recognize and compare two-dimensional images and three-dimensional objects of different colors by size was studied. The study was carried out on two species of monkey of different level of phylogenetic development: rhesus macaques (Macaca mulatta) and brown capuchin (Cebus apella). The monkeys were established to be able to recognize and to compare by size the two-dimensional pictures and three-dimensional objects of red, yellow, and green colors with identification of the relative sign “more.” Solution of visual tasks of the “more-less” type was achieved at any combination of stimuli regardless of color. The age-and specie-related differences were revealed during the course of training. In all monkeys the “transfer of learning” and formation of generalization were traced. This indicates a sufficiently well developed level of elementary thinking in the lower monkeys.