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A new public watermarking algorithm for RGB color image based on quantization index modulation (QIM) is proposed in this paper. The similarity of the corresponding coefficients produced by discrete cosine transform (DCT) of each red, green, blue channel shows that the margins of the corresponding coefficients of any two selected from the three color channels have proper distribution to be the signal constellation in QIM systems. Moreover, watermark embedded by modulated the margins to positive or negative range has obtained good balance between imperceptibility and robustness. Since the margins can be produced by any two of the three channels, the algorithm ensure its openness by keeping the record of the channels used to produce every margin the key for watermark extraction. Specific procedure and parameters selected are described in this paper. Experiments substantiate the practicability of the algorithm.