Using a dark brown, drybrush all the skulls and mechanical areas (anything with detail, really).
 
 
 
 
 

This includes all the details on the back too.
 
 
 
 

Next, using a blue-grey, drybrush all the other flat areas, avoiding the area around the hubs / lights...
 
 
 
 

...because these get painted with a dark red for contrast.
 
 
 
 
 
 

Now use a light brown to drybrush all the skulls on the shrine, wherever they appear.
 
 
 

Next, mix up a rust wash and apply it to any crevices and recessed detail you like.  You can also add some rust wash to the hub lights- as long as it isn't too thick, it shouldn't block out the light.  I also like to run streaks of rust wash down from selected rivets.
 
 
 

Once the rust wash is dry, mix up a bronze color (brass and steel) and drybrush all the mechanical bits- lamp posts, pipes, rivets, etc.
 
 
 
 

Drybrush some gold or brass color onto the ridges surrounding the skull details.
 
 
 
 
 

Finally, drybrush the skulls white.  And you're done!