how to show histogram of RGB

and present 3channels simultaneously.

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/matlab/ref/histogram.html#buiynvy-13 to see how to plot multiple histograms

number_of_bins = randi(25);

R = Yourimage(:,:,1);
G = YourImage(:,:,2);
histogram(R(:), number_of_bins);
hold on
histogram(G(:), number_of_bins);