Consider the following code for iterating through for loop in images by using matlab. In this example we are setting GREEN Channel of this image to 0 :

>> A = imread('DesertSmall.jpg'); % reading image
>> s = size(A) % image size (first height, second width, third channels)
>> figure
>> subplot(1,2,1),imshow(A); title('Original Image');
>> for i = 1 : s(1) % for height of image
     for j = 1 : s(2) % for width of image
       A(i,j,2) = 0; % A(<height>,<width>,<channel>), GREEN channel to 0;
     end
   end
>> subplot(1,2,2),imshow(A); title('Extracted Green Channel');

Output :