You can change the color of the wireframe by adding the objects to different layers (as you did), you can also change the color in the Drawing Overrides or in the Settings/Preferences Color menu.
One way to make the wireframe thicker is to only use the alpha from the rendered wireframe and then grow it by any number of pixels in your composting program.
This is not always a good way but you can try it, it should work on stills.