This is a very difficult problem. I doubt thresholding would work great but you can try the Color Thresholder on the Apps tab of the tool ribbon. If color alone is not enough to distinguish regions then you might have to add in other things like texture, stdfilt
It's a generic, general purpose demo of how to threshold an image to find blobs, and then measure things about the blobs, and extract certain blobs based on their areas or diameters.
If you know that a picture definitely has all 3 materials in it you can use k-means, or linear discriminant analysis on the image. Or if you can hand pick some samples then you can try KNN. I'm attaching demos for those you can adapt. I doubt any of them will be great but it's worth a try. Otherwise you could always try hand tracing the regions -- might not be acceptable if you have more than 100 images though. I'm attaching some drawing/masking demos also.
