How to convert a color of image to green?

I have this code for converting it to red

if true
  I= imread('1.tif');
R = I(:,:,1);
image(R), colormap([[0:1/255:1]', zeros(256,1), zeros(256,1)]), colorbar;
end
I changed the number 1 to 2 or 3 I got this message 
Index in position 3 exceeds array bounds (must not exceed 1).

Answer 1

[Jan]

The error message means, that the imported image is a grey scale image, such that I is a 2D matrix, not a 3D RGB array. If it is not an indexed image, the colorbar does not affect anything.

Try this:

R = cat(3, zeros(size(I), I, zeros(size(I));
image(R)