what does the colon mark indicates in the color seperation of image?

what does the colon mark indicates in the color separation of image Z(:,:,1)

Answer 1

[David Sanchez]

It says you are taking all the elements.

example. If

Z = [1 2 3;
     4 5 6;
     7 8 9];

Then

Z(:,1) = [1;
          4;
          7];
Z(2,:)  = [4 5 6];