This script is a demo that shows you how to find objects of a certain color in an image. For this example I'm using the standard onion, peppers, or kids image that ships with the Image Processing Toolbox. You can also select one of your own color images instead if you wish. Then I separate the image into its component hue, saturation, and value color bands. I compute thresholds, and ultimately find a mask for the yellow objects. Prompts will guide you through the demo step by step. I multiply this mask by the original image to show the image with only the yellow objects showing and everything else blacked out. This script can be adapted to find other colors (white, green, blue, yellow, etc.)
If you want to do any measurements of the objects, I refer you to my blob measurement demo file at http://www.mathworks.com/matlabcentral/fileexchange/25157
Requires the Image Processing Toolbox. Tested with MATLAB release R2010a.