Color space conversion is an important kernel in multimedia codecs such as JPEG and MPEG. When implemented using SIMD instructions, however, the performance improvement is often limited due to two reasons. First, corresponding color space components are stored at non-unit strides and, second, intermediate results can be larger than 8 bits. In this paper we show that extended subwords and the Matrix Register File (MRF) can be employed to mitigate these limitations. These techniques avoid rearrangement instructions and increase the number of subwords that are processed in parallel. Experimental results have been obtained by extending the SimpleScalar toolset. The results show that extended subwords and the MRF yield a speedup of up to 2.45x and 1.78x over MMX for the RGB-to-YCbCr and YCbCr-to-RGB kernels, respectively. Compared to C implementations, speedups of up to 10.09x and 6.74x, respectively, are obtained. Additionally, the results show that the speedup over MMX is higher for low issue rates. This means that extended subwords and the MRF are suitable techniques for embedded multimedia systems where high issue rates and out-of-order execution are too expensive. The results also show that using more registers improves performance substantially.