If you want to zero out the red and blue channels of your RGB image, you can do this:
rgbImage(:, :, [1, 3]) = 0;
imshow(rgbImage);
The reason that it was appearing gray is that the red and blue channels are the same as the green channel. Zeroing them out will make it look green.
Alternatively you can extract the green channel and apply a colormap.
rgbImage = imread('peppers.png'); % Sample image.
greenChannel = rgbImage(:, :, 2); % Extract green channel
z = zeros(256, 1);
ramp = (0:255)' / 255;
cmap = [z, ramp, z];
colormap(cmap);
colorbar;
imshow(greenChannel, cmap);
