How can i use parfor in my code?

I get an error saying "The variable rgb in a parfor cannot be classified." when i use "parfor" instead of for

here is my code

for m=1:size(rgb,1)
    for n= 1:size(rgb,2)

          for mm=1:size(t1rgb,1)
             for nn= 1:size(t1rgb,2)

          if abs(rgb(m,n,1)-t1rgb(mm,nn,1))<2 & abs(rgb(m,n,2)-t1rgb(mm,nn,2))<2 & abs(rgb(m,n,3)-t1rgb(mm,nn,3))<2
           gray = .3*rgb(m,n,1)+.59*rgb(m,n,2)+.11*rgb(m,n,3);
           rgb(m,n,:)= [gray gray gray];

          end 
             end
          end
      end
  end

[Walter Roberson]

I really recommend that you vectorize your code.

Do I understand correctly that for each given pixel location, if the pixel is within (+/-2, +/-2, +/-2) for any pixel in t1rgb, that you want to replace the rgb pixel with a gray scale version of itself ?? If so then you could vectorize the test by using ismembertol() with an absolute tolerance configured.