The number of discernible object colors is estimated over a wide range of color temperatures and illuminance levels using several chromatic adaptation models, color spaces, and color difference limens. Through comprehensive simulation, it is found that there are some limitations to current color appearance models for estimating the number of discernible colors. The fundamental problem lies in the von Kries-type chromatic adaptation transforms that indifferently affect the ranking of the number of discernible colors at different color temperatures.