In the field of color images watermarking, many methods are accomplished by marking the image luminance, or by processing each color channel separately. This paper proposes a new DCT domain watermarking expressly devised for RGB color images based on the diversity technique in communication system. The watermark is hidden within the data in the same sequence by modifying a subset of block DCT coefficients of each color channel. Detection is based on a combination method by taking into account the information conveyed by three color channels. Even if a particular channel is severely faded, we may still be able to recover a reliable estimated of transmitted watermark through other propagation channel. Experimental results, as well as theoretical analysis, are presented to demonstrate the validity of the new approach with respect to algorithm operating on image luminance only.