Instability in the color appreciation of monochromatic flashes for a normal trichromat is studied with respect to the instability in the stimulus due to the statistical fluctuation in the number of incident quanta and the distribution of this number over the retinal receptors. The frequencies of reddish, yellowish, greenish, bluish, colorless, white, etc., appearances in the perception were determined under variation of brightness, wavelength, exposure time, and area of the test stimulus. By comparison with the quantum theoretical studies conducted previously for absolute threshold values, it is found that for green excitation, three quanta, and for red, two quanta are required for a colored perception. For the long wave end two systems are sensitive, a “red” and a “colorless” system. In the green two quanta excitations result in colorless appearances of the perception.