how to visualize image segmentation using matlab
some pointers for you to figure out the solution:


you can convert segmentation results to CONTOURS and then plot contours, with thick boundary lines.
first convert the image to grayscale,

x = rgb2gray(x);
img(:,:,1) = x;
img(:,:,2) = x;
img(:,:,3) = x;

now, for each cluster you can modify the color for that part of image. say for example row1, col1, keep the index for class 1:
  for i=1:size(row1,1)
        img2(row1(i), col1(i), 1) = 200;
    end
in the end show image with modified colors.
show image, then hold on, then show ONLY one cluster at a time as a region filled with a solid color, then set transparency to 50%.

two ways 

to set transparencey: 
set(h, 'AlphaData', alpha_data); % e.g. alpha_data = 0.5
or 
h=imagesc(img2);
alpha(h,0.5);