The colored shadow phenomenon of polka-dotted patterns is investigated by applying image analysis. The purpose of our study is to evaluate how the achromatic color of polka-dots is perceived as having a slightly yellow hue which is the complementary color of blue when placed in a blue background. RGB images are converted to lightness ( L ^*) images. Each pixel of L ^* image is assigned a gray-level value from 0 for black to 255 for white. The co-occurrence contrast (CON) is calculated for L ^* images. CON as a parameter for lightness contrast effect increases with the increase of the difference of lightness between the polka-dotted pattern (the test field) and the background (the inducing field), regardless of whether the lightness of the figure becomes higher or lower than that of the background. The degree of colored shadow of 30 kinds of polka-dotted patterns is evaluated by sensory evaluation, according to the rating-scale method, and the relation between their sensory-evaluation values and CON is examined. It is concluded that the colored shadow of polka-dotted patterns is enhanced with the decrease of the lightness contrast effect.