How to change pixel intensity in a color picture

I have a color image in which I want to reduce the intensity of the leftmost vertical third of the picture by 100, and increase the rightmost vertical third of the picture by 100. (This is for a class project, btw.) I've tried implementing the below code, but all it seems to do is make it more or less red, as opposed to lighter or darker. How can I fix this? edit: The function takes in an image array, img, where the values are from 0 to 255.

function [quant] = imgChange(img)
  quant = img;
  s = size(img);
  third_left = round(s(2)/3);
  third_right = third_left * 2;
  quant(:,1:third_left) = quant(:,1:third_left) - 100;
  quant(:,third_right:s(2)) = quant(:,third_right:s(2)) + 100;
end

Answer 1

[Guillaume]

Well, yes, if your image is RGB, you have 3 colour planes in the third dimension. You need to tell matlab that you want to affect all three, so:

quant(:,1:third_left, :) = quant(:,1:third_left, :) - 100;
quant(:,third_right:s(2), :) = quant(:,third_right:s(2), :) + 100;