I have noticed that my Sigma cameras have the ability to differentiate subtle changes in in color better than my Bayer cameras. I notice it mostly in greens. My SD15 also seems to be a bit more color accurate than my Pentax and Sony cameras.
This is difficult to understand since it is known that the color separation of the foveon chip is not as efficient as the Bayer color filter. Most all reviews of Sigma cameras also talk about the great colors, IQ, and saturation at low ISO.
My only guess is that the broad spectrum sensitivity of each layer helps differentiate color at the expense of color noise at lower exposure levels. This could be achieved with the Bayer filters also but then the noise would be similar to the the Foveon. Overall public acceptance of higher speed must be more important than outright IQ.