White gouache (applied with a flexible crow-quill) on black charcoal paper, with a PS tweak or 2.
Edit: Ben asked me about the pen & gouache technique so I thought I'd add a few tips.
First, mix a small quantity of gouache to a light-creamy consistency (I use a clean empty ink bottle since I always have those handy), then apply the gouache to the underside of the nib with an old brush (it's bad for the brush so don't use your good ones).
Keep a water jar handy for when the gouache starts to dry on the nib-- just touch the nib to the surface of the water & it will make it flow again. Keep scrap paper handy to get the flow started when it's stubborn, & be sure to shake the nib after each load of paint or you may be unhappily surprised with a sudden blob. If you have any more questions just let me know!