That would work if you wanted a red image. It would even work if your input image were single-channel.
inpict = imread('cameraman.tif'); % MxNx1
inpict(:,:,2:3) = 0; % create red image by implicit expansion
imshow(inpict)
... but it wouldn't work if you wanted to do a green or blue image using the same method.
inpict(:,:,[1 3]) = 0; % create green image the same way?
imshow(inpict) % it's black because we just zeroed channel 1
See this thread:
