They look gray scale, so let's convert to gray scale first, and then do the copying
[rows, columns, numberOfColorChannels] = size(rgbImage);
% Get grayscale image from original RGB image.
if numberOfColorChannels == 3
grayImage = rgbImage(:,:, 2); % Extract green channel only - it's usually the least noisy one.
else
grayImage = rgbImage;
end
image2 = zeros(rows, columns); % Not sure why you did this because it's not necessary.
% Now copy gray image into image2
image2 = grayImage;
A contrast difference could happen if you used different parameters for imshow. For example
imshow(grayImage, []); % Stretch contrast
imshow(image2); % No contrast stretch.