how to extract unique pixels from on image?

>> I = randi(7,5,4,3) % MxNx3 image array.
I(:,:,1) =
   3   2   5   5
   7   3   6   3
   6   7   6   4
   5   6   4   5
   1   1   1   6
I(:,:,2) =
   7   2   5   2
   7   4   5   7
   2   2   1   4
   3   7   7   2
   6   3   7   7
I(:,:,3) =
   1   2   2   1
   7   2   6   1
   5   6   4   7
   7   6   5   7
   4   6   3   3
>> U = unique(reshape(permute(I,[3,2,1]),3,[]).','rows') % unique pixels (each row = 1 pixel rgb)
U =
   1   3   6
   1   6   4
   1   7   3
   2   2   2
   3   4   2
   3   7   1
   4   4   7
   4   7   5
   5   2   1
   5   2   7
   5   3   7
   5   5   2
   6   1   4
   6   2   5
   6   5   6
   6   7   3
   6   7   6
   7   2   6
   7   7   7