The present paper reviewed experiments, especially the methods used therein, that investigated colors seen by other persons, color-abnormal persons, including unilateral color-blind and cataract patients. Binocular color-matching between their abnormal and normal eyes provided very interesting information concerning colors seen by abnormal eyes. Multi-dimensional scaling applied to color-difference judgements of color-normal and -abnormal observers produced different two-dimensional mappings of colors similar to distorted color circles. Apparent depths of various color surfaces were also compared between the two kinds of observers. The paper also reviewed some other experiments on colors observed by bees, carp, monkeys and a chimpanzee. These were studied using color-chips and color-lights. Careful control of the stimuli was necessary, because some creatures, like bees, can see some colors in wavelengths where humans cannot. Experiments on a chimpanzee indicated that it could learn various color categories in a similar manner as humans, even when only a typical sample of each color was presented during the learning period.
