See my Image Segmentation Tutorial. http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862
You need to first segment the image to get a binary image then you can call regionprops(). Whether you call rgb2gray() or just extract one of the RGB color channels, or use rgb2hsv() or rgb2lab(), or PCA or something else really depends on your image and what you need to find in it. Post your image(s) if you want more advice.
