This is Day 33. The main lines were done with a 2B, the darker shading was a 4B. I've come to love the 4B as a main tool, but it's good to have a variety.
Harder leads produce very faint, easily erasable lines if you press lightly on them, which makes them great for skeletoning. H pencils also make good silvery metallic colours. A set of pencils will cost you maybe $10, and just play around with them until you get their measure.