Here's the demo:
After that, if you need spatial centroids (x,y,z locations) in addition to color centroids (the RGB values of the color clusters' centroids), it's a simple matter to find centroids using bwlabel() and regionprops(), like in my BlobsDemo ( http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862 ).
