please, how do we give each region a color after the segmentation ?

Answer 1

[Guillaume]

label2rgb is one way to do it.

Answer 2

[Image Analyst]

Try this:

labeledImage = bwlabel(binaryImage, 8);     % Label each blob so we can make measurements of it
% labeledImage is an integer-valued image where all pixels in the blobs have values of 1, or 2, or 3, or ... etc.
imshow(labeledImage, []);  % Show the gray scale image.
title('Labeled Image, from bwlabel()', 'FontSize', captionFontSize);

% Let's assign each blob a different color to visually show the user the distinct blobs.
coloredLabels = label2rgb (labeledImage, 'hsv', 'k', 'shuffle'); % pseudo random color labels

where "binaryImage" is your segmented image, or possibly you've already labeled it and labeledImage is your segmented image.