[N,M,K] = size(I)

N =

   480


M =

   640


K =

     3



figure(1)

r=1:10:N

r =

  Columns 1 through 12

     1    11    21    31    41    51    61    71    81    91   101   111

  Columns 13 through 24

   121   131   141   151   161   171   181   191   201   211   221   231

  Columns 25 through 36

   241   251   261   271   281   291   301   311   321   331   341   351

  Columns 37 through 48

   361   371   381   391   401   411   421   431   441   451   461   471


c = 1:10:M;


I1 = I(r,c,:);


figure, imshow(I1);
T = 100;

ind = find(I(:,:,3)>T);
whos ind
  Name           Size              Bytes  Class     Attributes

  ind       228384x1             1827072  double              



I0 = I;
I0(:,:,:) = 0;
I0(ind) = 255;


figure, imshow(I0);
I2 = I0;



I2(:,:,2) = I0(:,:,1);
figure, imshow(I2);
I2(:,:,3) = I0(:,:,1);

figure, imshow(I2);

I0(ind) = 100;

I0(ind+80*640) = 100;
I0(ind) = 100;
I0(ind+480*640) = 100;

I0(ind+2*480*640) = 100;

figure, imshow(I0);
figure, imshow(I0);