I am trying to replicate the functionality that Dribbble.com does with detecting the predominant colors in an Image. In the image below you can see a screenshot from Dribbb
I have a Unix bash shell script with ImageMagick called dominantcolor that may do what you want. See my scripts web site at http://www.fmwconcepts.com/imagemagick/index.php. You an run it from PHP exec(). See my pointers for use on my home page.
Input:
dominantcolor -n 6 -p all -s save plate.png
count,hexcolor
586,#5ECADC
520,#AFA85D
469,#3C3126
462,#B9C8BB
258,#488A70
205,#B06928
The -n 6 is the desired number of colors in the color quantization. The -p all means print all counts and colors for the resulting 6 colors. The -s save indictates to save a swatch image.
Colors below are shown with the dominant color on the left and decreasing count colors towards the right according to the list above.