This is the bone shader I created:

The Diffuse node that is part of the AO setup has a dark brown color chip to simulate dirt in cracks & crevices, as well as the deep recesses of the object. The Ambient Occlusion node Distance channel is set high to capture more shadow detail away from the surface, almost simulating a remnant of skin tissue. Its color chip was set to green to also enhance details, but any color would do. Throughout the shader, the RGBRamps control the intensities and details from their associated texture nodes.