adding an RGB volume to a grayscale MRI volume

I have a grayscale MRI volume of dimension 160x216x176.

I also have a volume corresponding to its labels, which is in RGB, so its 160x216x176x3.

I want to use imshow to show each mri volume slice superimposed to its labels, with the mri volume appearing in gray and the labels appearing in a custom colormap that I created.

I use this to produce the RGB slices:

   labelRGB_slice=label2rgb(handles.label(:,:,handles.current_slice_z), my_colormap, [0 0 0]);
    handles.labelRGB(:,:,handles.current_slice_z, : , : , :) = labelRGB_slice;

this is the imshow that I plan to use to update axes:

 imshow((handles.imdata_show(:,:, handles.current_slice_z) +...
           handles.labelRGB(:,:,handles.current_slice_z, : , : )),... 
           'DisplayRange', [])

it's obviously faulty cause the matrix dimensions don't agree, but I don't know how to fix the grayscale slice to make it addable to the rgb slice.

What should I use here? repmat? ind2rgb?

I tried using ind2rgb with a 'gray' colormap, like this:

   imshow((ind2rgb(handles.imdata_show(:,:, handles.current_slice_z), gray) +...
              squeeze(handles.labelRGB(:,:,handles.current_slice_z, : , : ))),... 
              'DisplayRange', [])

and got this botched image, losing all gray contrast (rgb labels haven't been placed yet):

http://imageshack.us/photo/my-images/823/69bg.png