The latest print issue of Optical Engineering (June 2007) has an interesting paper on color image contrast enhancement based on mathematical morphology.
The authors use the CIE 1976 UCS diagram to convert the images to a luma-chroma representation. They use mathematical morphology on the Y plane, applying open and close operators to find the image's principal regions according to a morphological contrast criterion. They also have a second approach based on retinex theory, but using morphological operations instead of the usual family of Gaussian transformations. In the color channels they use the gravity center law to boost the contrast.
This paper can be found online in the SPIE Digital Library under the title "Morphological multiscale contrast approach for gray and color images consistent with human visual perception" or doi:10.1117/1.2749711.