Second, we have studied how color appearance depends on spatial pattern. The color appearance of a spatial squarewave pattern depends on the spatial frequency of the squarewave. We have found that the change in color appearance with spatial frequency can be explained by assuming that signals from three opponent-colors mechanisms are scaled by a gain factor that depends on the local spatial frequency content of the image. This model is consistent with common engineering practice, in which it is well known that color appearance becomes progressively desaturated as the spatial frequency of the stimulus increases. We have captured this qualitative observation in laboratory measurements and modeled the phenomenon quantitatively