Most image data that we encounter is in color, thus measuring clutter in color images has become increasingly important. The extension of phase correlation to quaternion space, which can measure color similarity as well as the structural similarity between two color images, is defined. It is used to describe the global clutter in color images. The correlation degrees between the experimental probability of detection and that predicted by the clutter metric are presented. Experiment results show that the quaternion phase-correlation-based clutter metric can perform well in quantifying color image clutter.