could you use a toon shader instead? toon shaders can create thick outlines around your main objects and also include as many (or as little) colors as you want in the main object. you might also be able to just have the shader outline the objects and not mess with the original materials on the interior of your characters if you combine them in a layered shader.

if you're just looking to find a way to make the layers visible, try changing the outline layer to template mode (that's the box next to the v in your display layer).