[filename, filepath] = uigetfile();
if ~ischar(filename); return; end %user cancel
fullname = fullfile(filepath, filename);
img = imread(fullname);
R = zeros(size(img), class(img));
G = R;
B = R;
R(:,:,1) = img(:,:,1);
G(:,:,2) = img(:,:,2);
B(:,:,3) = img(:,:,3);
imshow(handles.axes_for_RGB, img);
imshow(handles.axes_for_R, R);
imshow(handles.axes_for_G, G);
imshow(handles.axes_for_B, B);