what is the reason of this error"Field reference for multiple structure elements that is followed by more reference blocks is an error."

hi.i have RGB image and want to change the value of pixels that are zero into 255 .i use the follow code but this run just once and for i=1 and j=2 return this error "Field reference for multiple structure elements that is followed by more reference blocks is an error." why? how can i solve this error?

    S=size(imrgb);
num_rowgray=S(1,1);
num_colgray=S(1,2);
imr = imrgb(:,:,1);
img = imrgb(:,:,2);
imb = imrgb(:,:,3);
imcolor.r=[];
imcolor.g=[];
imcolor.b=[];
for i=1:num_rowgray
    for j=1:num_colgray
        imcolor(i,j).r=imr(i,j);
        imcolor(i,j).g=img(i,j);
        imcolor(i,j).b=imb(i,j);
if imcolor.r (i,j)==0 && imcolor.g (i,j)==0 && imcolor.b (i,j)==0 

            imcolor(i,j).r =255;
            imcolor(i,j).g =255;
            imcolor(i,j).b =255;

        else

            imcolor(i,j).r(i,j)=0;
            imcolor(i,j).g(i,j)=0;
            imcolor(i,j).b(i,j)=0;

        end
    j=j+1;
    end
    i=i+1;
end

Answer 1

[Stephen23]

There is no need to use loops and such complicated code. Logical indexing is much simpler:

X = all(imrgb==0,3);
imrgb(X(:,:,[1,1,1])) = 255;

And that is all.

[mandana]

thanks so much Stephen