There are several ways of doing this. See my attached demos on kmeans() and classify().
kmeans() is usually a bad way to classify, as you'll see. Plus it's not robust since how well it works depends on how many of the colors you want are in your photo. Better is to do color segmentation like I do in my demos in my File Exchange. https://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862&sort=downloads_desc
