A new architecture for color space conversion in RGB to YCbCr domain has been proposed. The architecture exploits the similarity in bit-planes of a natural image to bring the algorithmic efficiency of distributed arithmetic (DA) approach close to that of a full multiplier design. Modifications have been carried out in DA to further reduce the computational complexity by a factor of two for most natural images. The color space conversion architecture has been incorporated in OR1200 open source IP processor core to provide custom instruction. Simulation and synthesis results on Xilinx FPGA are provided.